fix(backend): apply install_env to install commands

[risu729]

Summary

• applies per-tool install_env to command-backed install paths across package-manager backends, vfox cmd.exec hooks, SPM build/probe commands, and core language install-time commands
• adds install_env docs for affected backend and language pages, including SPM shorthand and vfox helper caveats
• adds a vfox e2e covering install_env propagation into cmd.exec during install

Project item: https://github.com/users/risu729/projects/3/views/1?filterQuery=install_env&pane=issue&itemId=188373493

Tests

• cargo fmt --all -- --check
• git diff --check && git diff --cached --check
• CARGO_TARGET_DIR="$(cargo metadata --format-version 1 --no-deps | jq -r .target_directory)" mise run test:e2e -- test_vfox_install_env

[gemini-code-assist]

Code Review

This pull request introduces the install_env tool option across all mise backends, allowing users to specify environment variables for tool installation and build processes. The update includes comprehensive documentation for each backend and a new E2E test. Feedback indicates that the precedence of environment variables needs adjustment in several plugins: critical mise-managed variables (e.g., RUSTUP_TOOLCHAIN, DOTNET_ROOT, PIP_REQUIRE_VIRTUALENV) should be protected from user overrides to prevent corrupted installations, whereas user-defined variables should be allowed to override non-critical defaults like MAKEFLAGS or global CFLAGS.

[greptile-apps]

Greptile Summary

This PR propagates the per-tool install_env option into every command-backed install path across all package-manager backends, vfox cmd.exec, SPM build/probe commands, and all core language install-time commands, along with matching documentation and an e2e test for vfox.

• Core plumbing: A new ToolVersion::install_env() helper centralises access to the option; the SPM backend introduces a with_install_env wrapper (needed because duct::Expression has no .envs() method); the vfox backend merges install_env into cmd_env after dependency env, so it applies even when dependency resolution fails.
• Node refactoring: fetch_tarball is restructured to separate the lock-platform read from the mutable entry update, which was required to allow tv to be passed to get_checksum/verify_with_gpg without holding two borrows on tv.lock_platforms simultaneously — the logic is equivalent to the original.
• Env ordering: install_env is consistently inserted before mise-managed tool paths so those remain authoritative, matching the intentional pattern documented in previous review threads.

Confidence Score: 5/5

Safe to merge; the change is additive with no control-flow changes outside the deliberate node.rs borrow-checker refactor, which is logically equivalent to the original.

Every modified command site follows a consistent, reviewed ordering pattern. The structural refactor in node.rs preserves identical semantics. The vfox and pipx changes carry incidental correctness improvements. Previously discussed ordering trade-offs are acknowledged and intentional.

No files require special attention beyond what was already discussed in prior review threads.

Important Files Changed

Filename	Overview
src/toolset/tool_version.rs	Adds install_env() helper method on ToolVersion that clones the map from request.options() on every call; used uniformly across all call sites
src/backend/spm.rs	Introduces with_install_env helper (necessary because duct Expression lacks an envs() method) and applies it to all three swift command sites
src/backend/vfox.rs	Merges install_env into cmd_env after dependency env, correctly using unwrap_or_default() so install_env still applies when dependency_env errors
src/plugins/core/node.rs	Refactors fetch_tarball to read lock_platforms before passing tv to get_checksum to avoid borrow conflicts; adds install_env to all six command sites
src/backend/pipx.rs	Reorders env chain to add install_env and fixes existing bug where UV_TOOL_DIR and PIP_INDEX_URL could be overridden by ts.env_with_path_without_tools
src/plugins/core/erlang.rs	Restructures kerl command so install_env can override MAKEFLAGS but KERL_BASE_DIR remains authoritative
src/plugins/core/rust.rs	Adds install_env before exec_env on all three rustup/rustc command sites; exec_env still takes precedence for conflicting keys
src/plugins/core/dotnet.rs	Adds install_env before exec_env on both commands; exec_env remains authoritative by design
e2e/backend/test_vfox_install_env	New e2e test that creates an in-process vfox plugin and asserts install_env is visible inside PostInstall cmd.exec()
src/backend/npm.rs	Adds install_env before BUN/PNPM/NPM manager-managed path env vars on all four install paths

_{Reviews (5): Last reviewed commit: "fix(backend): remove dead cargo feature ..." | Re-trigger Greptile}