[publish images] Remove sccache, use ccache only (#25682)

## Summary
- Remove sccache support entirely, use ccache only
- Missing ccache no longer fails the build (just skips caching)
- Remove S3 distributed cache support

## Changes
- Remove `cmake/tools/SetupSccache.cmake` and S3 distributed cache
support
- Simplify `CMakeLists.txt` to only use ccache
- Update `SetupCcache.cmake` to not fail when ccache is missing
- Replace sccache with ccache in bootstrap scripts (sh, ps1)
- Update `.buildkite/Dockerfile` to install ccache instead of sccache
- Update `flake.nix` and `shell.nix` to use ccache
- Update documentation (CONTRIBUTING.md, contributing.mdx,
building-windows.mdx)
- Remove `scripts/build-cache/` directory (was only for sccache S3
access)

## Test plan
- [x] Build completes successfully with `bun bd`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

flake.nix

@@ -40,7 +40,7 @@
           pkgs.cmake # Expected: 3.30+ on nixos-unstable as of 2025-10
           pkgs.ninja
           pkgs.pkg-config
-          pkgs.sccache
+          pkgs.ccache
 
           # Compilers and toolchain - version pinned to LLVM 19
           clang