fix(backend): pass install_env to postinstall

[risu729]

Summary

• passes per-tool install_env into tool-level postinstall hook commands
• documents that tool-level postinstall receives install_env variables
• extends the postinstall environment e2e to cover install_env

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
• CARGO_TARGET_DIR="$(cargo metadata --format-version 1 --no-deps | jq -r .target_directory)" mise run test:e2e -- test_hooks_postinstall_env

[greptile-apps]

Greptile Summary

This PR fixes the tool-level postinstall hook so that per-tool install_env variables are passed into the postinstall execution environment, matching the behaviour users would already expect from seeing those variables during the install phase itself.

• src/backend/mod.rs: One-line fix in run_postinstall_hook — calls env_vars.extend(tv.request.options().core.install_env) after the config env block, so install_env entries are present (and override any conflicting earlier entries) when the postinstall command runs.
• e2e test: The existing test_hooks_postinstall_env script is extended with an INSTALL_ENV_VAR set via install_env and a new assertion that verifies it is visible inside postinstall.
• Docs: docs/configuration.md, docs/dev-tools/index.md, and docs/hooks.md are all updated to document that install_env variables are available during tool-level postinstall.

Confidence Score: 5/5

Safe to merge — a minimal, well-tested one-line fix with matching documentation and a new e2e assertion.

The change is a single extend call that mirrors the same pattern already used when applying install_env during the install phase (e.g. asdf.rs). Priority semantics are consistent: install_env uses extend (overrides), while global config env uses or_insert (does not override), which is the expected ordering. The e2e test directly exercises the new code path and the documentation is fully updated.

No files require special attention.

Important Files Changed

Filename	Overview
src/backend/mod.rs	Adds env_vars.extend(tv.request.options().core.install_env) in run_postinstall_hook so that per-tool install_env variables are propagated into the postinstall environment, consistently overriding earlier env entries.
e2e/config/test_hooks_postinstall_env	Extends the existing postinstall environment e2e test to include an INSTALL_ENV_VAR set via install_env and verifies it is visible during postinstall.
docs/configuration.md	Updates the install_env bullet to note it also applies to tool-level postinstall.
docs/dev-tools/index.md	Adds a note that install_env variables are available during postinstall, alongside the existing MISE_TOOL_INSTALL_PATH mention.
docs/hooks.md	Adds a bullet to the tool-level postinstall environment variable list noting that install_env variables are injected.

_{Reviews (2): Last reviewed commit: "fix(backend): pass install_env to postin..." | Re-trigger Greptile}

[gemini-code-assist]

Code Review

This pull request updates the documentation and implementation to ensure that environment variables specified in the install_env configuration are available during the postinstall hook. It includes a new end-to-end test to verify this behavior. A review comment pointed out a compilation error in the backend logic where a map was being extended with a reference instead of owned values, and a code suggestion was provided to fix it.