QVAC-17408: remove ccache from prebuild workflows

[jpgaribotti]

What problem does this PR solve?

• ccache setup across the eight prebuild workflows is duplicated and drifted across three incompatible patterns (ARM64-only with broken cache keys, hashFiles(vcpkg.json)-keyed combined cache, and github.sha-keyed split restore/save), adding maintenance load and review noise.
• Observed hit rates on these prebuild jobs no longer justify the cost of maintaining the ccache install / configure / cache blocks across Linux, macOS, and Windows.

How does it solve it?

• Removes ccache entirely from every prebuilds-*.yml workflow instead of standardizing it.
• Deletes, in each in-scope workflow: Install ccache on Linux / ...on macOS / ...on Windows, Configure ccache, Get / Restore / Save ccache cache (both combined actions/cache and split actions/cache/restore + actions/cache/save), Show ccache stats, and the CMAKE_C_COMPILER_LAUNCHER / CMAKE_CXX_COMPILER_LAUNCHER exports to $GITHUB_ENV.
• Drops the -D CMAKE_C_COMPILER_LAUNCHER=ccache -D CMAKE_CXX_COMPILER_LAUNCHER=ccache flags on bare-make generate in prebuilds-qvac-lib-infer-onnx.yml and prebuilds-ocr-onnx.yml.
• Preserves unrelated Windows bootstrap that shared the ccache step (git config --system core.longpaths true, choco upgrade llvm -y --no-progress) as their own Windows-only step.
• Extracts the onnx-tts brew unlink fmt || true fmt header-conflict workaround into its own macOS-only step so it survives the ccache removal.
• In-scope workflows: prebuilds-qvac-lib-infer-whispercpp.yml, prebuilds-qvac-lib-infer-parakeet.yml, prebuilds-qvac-lib-infer-onnx-tts.yml, prebuilds-qvac-lib-infer-onnx.yml, prebuilds-ocr-onnx.yml, prebuilds-qvac-lib-infer-llamacpp-llm.yml, prebuilds-qvac-lib-infer-llamacpp-embed.yml, prebuilds-lib-infer-diffusion.yml.
• Out of scope: cpp-test-coverage-qvac-lib-infer-parakeet.yml, benchmark-qvac-lib-infer-nmtcpp.yml, benchmark-ocr-onnx.yml — these are not prebuild workflows and still use ccache; they can be revisited separately.

How was it tested?

• YAML parse-checked locally for all 8 edited workflow files.
• Verified zero residual references to ccache, CMAKE_C_COMPILER_LAUNCHER, or CMAKE_CXX_COMPILER_LAUNCHER in any prebuilds-*.yml.
• Ran the prebuilds for each addon, all completed in the expected time.

[github-actions]

Tier-based Approval Status

**PR Tier:** TIER1

**Current Status:** ✅ APPROVED

**Requirements:**
- 1 Team Member approval ❌ (0/1)
- 1 Team Lead OR Management approval ✅ (3/1)

**Bypass rule:** Triggered (2+ Team Lead approvals (Tier 1 exception)). This PR is approved regardless of tier.

---
*This comment is automatically updated when reviews change.*

[jpgaribotti]

/review