PERF: Add ccache/sccache support to GitHub Actions CI workflows

[hjmjohnson]

Summary

• Add hendrikmuhs/ccache-action@v1.2 to pixi.yml (ubuntu, windows, macos) and arm.yml (ubuntu-arm, macos rosetta, macos Python) to cache compiled objects between CI runs
• Add configure-ci pixi task in pyproject.toml that passes -DCMAKE_C_COMPILER_LAUNCHER=ccache and -DCMAKE_CXX_COMPILER_LAUNCHER=ccache to CMake; pixi.yml uses this task instead of configure
• arm.yml injects the same cmake launcher variables via dashboard_cache so the CTest dashboard script picks them up at configure time
• Cache sized at 11G to accommodate the full ITK object file footprint; keyed on OS + pixi.lock hash (pixi workflow) or OS + matrix name (arm workflow) to avoid stale cache on toolchain upgrades

Test plan

• Verify CI runs complete successfully on first run (cold cache — expect no speedup)
• Verify subsequent CI runs show ccache hit rate > 0% in the ccache-action summary
• Confirm no build failures introduced by the compiler launcher on all three OS platforms in pixi.yml
• Confirm arm workflow builds succeed with ccache launcher on ubuntu-arm, macos rosetta, and macos Python matrix entries

🤖 Generated with Claude Code

[hjmjohnson]

On my local computer, using ccache GREATLY accelerates the build process when only a single test is modified. Touching the CMakeLists.txt files often triggers the need to rebuild 6760 object files, which takes about 30 minutes; with CCache, it is done in about 2 minutes (with mostly cache hits). I think this will be the case for many CI builds if we can get this to work.

[dzenanz]

Looks good.

[hjmjohnson]

NOTE: Compiler object caching with very few changes can reduce the build time to under 10 minutes for builds that previously took 2 hours.

[dzenanz]

Sounds like a great improvement.

[blowekamp]

This is a great addition!!! I used to use it with CircleCI with SimpleITK, but never reimplemented in with GitHub Actions.

Please check the documentation for the hash keys, there is some tuning that can be done to better get a good cache.

We have had some disk space issues on several builds. Maybe it was just the Python builds? This may cause problems with them. It may take a couple runs for the cache to get filled, and the disk space to be come an issue. Are there any stats on the current disk usage being displayed?

[blowekamp]

The documentation of the hash key is here:
https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching

The has key is matched from the beginning to the end, so it should start with the less specific an go to the more specific. I suggest the following:

ccache-v1-${{ matrix.os }}-${{ matrix.name }}-${{branch-name}}

Additionally with the raw action/cache there is a restore key field that can take multiple lines. This should have the above first then ccache-v1-${{ matrix.os }}-main so that is falls back to the cache on the main. I don't recall the more advance logic to get the "release" cache to work correctly for PR to the release branch.... maybe a variable for the target branch name would be appropriate as a heigher priority.