refactor: simplify ccache to single directory with restore-keys fallback

Removed split ccache configuration (~/.ccache-main + ~/.ccache-current).

The split had an edge case: building a feature branch before main branch
cache is primed results in double ccache disk usage:
- Feature branch builds first → populates ~/.ccache-current (2G max_size)
- Main branch builds later → populates ~/.ccache-main (2G max_size)
- Next feature branch build → restores both caches (4G total)

This doubles ccache disk usage unnecessarily on GitHub-hosted runners.

New single-directory approach:
- Single ~/.ccache directory for all branches
- Branch-specific keys with restore-keys fallback to main branch
- Cache actions handle cross-branch sharing via partial-match mode
- Simpler and prevents double disk usage edge case

Changes:
- xahau-configure-ccache: Single cache_dir input, removed branch logic
- xahau-ga-build: Single restore/save step, uses restore-keys for fallback
- xahau-ga-nix: Removed is_main_branch parameter

Disk usage: 2G max (vs 4G worst case with split)

.github/workflows/xahau-ga-nix.yml

@@ -237,7 +237,6 @@ jobs:
           max_size: 2G
           hash_dir: true
           compiler_check: content
-          is_main_branch: ${{ github.ref_name == env.MAIN_BRANCH_NAME }}
 
       - name: Configure Conan
         run: |