perf(ci): re-enable compiler cache for Windows x64 builds

[BenJule]

Problem

Windows x64 is the only platform without a compiler cache. After the ccache fixes (#449, #450), Linux and macOS builds drop from ~60 min / ~150 min to ~15-20 min / ~35-45 min with warm caches. Windows x64 stays at ~60 min and is now the bottleneck for create_release (which waits on Debian + Ubuntu + Windows x64).

sccache was removed entirely in PR #433 because it produced 0% hit rate and caused C1041 PDB contention (mozilla/sccache#2320) under Ninja + /MP parallel builds.

Goal

Re-enable a working compiler cache for Windows x64 so it builds in ~15-20 min like the other platforms, removing the last bottleneck from the release pipeline.

Approach to try

The documented sccache + MSVC setup (already partially in place via PR #432) is:

• -DCMAKE_POLICY_DEFAULT_CMP0141=NEW
• -DCMAKE_MSVC_DEBUG_INFORMATION_FORMAT=Embedded (emits /Z7, no shared .pdb → no C1041)

This is already set in _build-core.yml. What's missing / to investigate:

1. Re-add mozilla-actions/sccache-action + SCCACHE_GHA_ENABLED and -DCMAKE_C/CXX_COMPILER_LAUNCHER=sccache for the x64 (Ninja) build only — NOT ARM64 (VS generator ignores the launcher).
2. Verify the GHA cache backend initialises before the MSVC toolchain (the original 0% hit rate came from sccache starting after cl.exe was already resolved). May need to start sccache earlier in the job.
3. Apply the same CCACHE_IGNOREOPTIONS equivalent for sccache — exclude SLIC3R_DEV_TIMESTAMP from the hash (sccache: rewrite the compile command or use SCCACHE_* filtering), otherwise the timestamp busts every entry like it did for ccache.
4. Confirm /Z7 (embedded debug info) holds under parallel Ninja so C1041 stays gone.

Reference

• sccache README (MSVC section): https://github.com/mozilla/sccache
• C1041 issue: Issue: fatal error C1041 During Parallel Build with cl.exe mozilla/sccache#2320
• Our ccache timestamp fix (same root cause to handle): PR fix(ci): correct ccache variable CCACHE_IGNOREOPTIONS (was non-existent REGEX variant) #449 (CCACHE_IGNOREOPTIONS=-DSLIC3R_DEV_TIMESTAMP=*)

Acceptance criteria

• Windows x64 build shows >80% sccache hit rate on a warm run
• No C1041 errors under parallel build
• Windows x64 release build time drops to ~15-25 min

Note

OrcaSlicer and PrusaSlicer both use the VS generator (no sccache) on Windows, so there's no upstream reference — this would be ahead of both.

[github-actions]

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