ci: make MRBind build cache non-fatal

[Fedr]

Summary

Make actions/cache@v5 non-fatal inside the build-mrbind composite action — a silent cache-service crash should not abort the whole job.

Motivation

On PR #6121, the windows-build-test (msvc-2019, Release, CMake, x64-windows-vs2019-meshlib) job in run 26152206642 failed at the Build MRBind composite step in ~2 seconds with zero log output. Tracing the timeline:

• Get MRBind submodule SHA succeeded
• actions/cache@v5 started at 09:00:14.67Z, the next workflow step's group started at 09:00:15.25Z — a 0.57 s gap with no output at all, while the two earlier actions/cache@v5 invocations in the same job (CUDA cache, MSYS2 cache) both printed Cache restored successfully as expected
• the composite was marked failure, so Generate C bindings, Build, tests, archive, upload — all skipped

Looks like a transient cache-service blip causing the action to crash before its logger flushes anything. Even with debug logging enabled we'd still pay a full job failure for what should be a recoverable event.

Fix

One-line continue-on-error: true on the cache step. On a cache-service error:

• the composite continues
• steps.cache.outputs.cache-hit is empty → the existing if: steps.cache.outputs.cache-hit != 'true' guards on the platform-specific Build MRBind sub-steps fire → MRBind rebuilds from scratch
• install_mrbind_windows_msys2.bat (and the Unix equivalent) wipes build/ before rebuilding, so a partially restored cache dir is safe
• the post-action save still runs and repopulates the cache, so the next run is back to cache-hit speed

Worst-case cost on a transient failure: one rebuild (~10 min on Windows, less elsewhere) instead of a hard job failure that takes the entire matrix entry down.

Test plan

• CI green across all platforms that use build-mrbind (run 26155116273): 28 success, 11 skipped, 0 failures across Windows (msvc-2019 + msvc-2022, Debug + Release), Linux vcpkg (x64 + arm64, Debug + Release, GCC 11 + Clang 21), Ubuntu (22.04 + 24.04, arm64 + x64, GCC 12 / 13 / 14), macOS (arm64 Debug + Release, x64 Release), and emscripten (singlethreaded + multithreaded + multithreaded-64bit, build + test).
• Cache-hit path still short-circuits the build. On the exact matrix entry that failed silently on Planar filling with plan #6121 (windows-build-test (msvc-2019, Release, CMake, x64-windows-vs2019-meshlib), job 76932415924): the MRBind cache key was unchanged by this PR (only action.yml was touched, not the cache-key inputs), so the step hit cache (Cache hit for: mrbind-build-windows-clang18-d5ca6d...), the conditional Build MRBind (Windows) sub-step was skipped, and the next workflow step (Generate C bindings) immediately invoked the cached mrbind / mrbind_gen_c binaries. Post-action correctly logged Cache hit occurred on the primary key ..., not saving cache.