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/actions/xahau-ga-build/action.yml

@@ -65,25 +65,12 @@ runs:
         SAFE_BRANCH=$(echo "${{ github.ref_name }}" | tr -c 'a-zA-Z0-9_.-' '-')
         echo "name=${SAFE_BRANCH}" >> $GITHUB_OUTPUT
 
-    - name: Restore ccache directory for default branch
+    - name: Restore ccache directory
       if: inputs.ccache_enabled == 'true'
       id: ccache-restore
       uses: ./.github/actions/xahau-actions-cache-restore
       with:
-        path: ~/.ccache-main
-        key: ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ inputs.main_branch }}
-        restore-keys: |
-          ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-
-          ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-
-        aws-access-key-id: ${{ inputs.aws-access-key-id }}
-        aws-secret-access-key: ${{ inputs.aws-secret-access-key }}
-
-    - name: Restore ccache directory for current branch
-      if: inputs.ccache_enabled == 'true' && steps.safe-branch.outputs.name != inputs.main_branch
-      id: ccache-restore-current-branch
-      uses: ./.github/actions/xahau-actions-cache-restore
-      with:
-        path: ~/.ccache-current
+        path: ~/.ccache
         key: ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ steps.safe-branch.outputs.name }}
         restore-keys: |
           ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ inputs.main_branch }}
@@ -169,20 +156,11 @@ runs:
       shell: bash
       run: ccache -s
 
-    - name: Save ccache directory for default branch
-      if: always() && inputs.ccache_enabled == 'true' && steps.safe-branch.outputs.name == inputs.main_branch
+    - name: Save ccache directory
+      if: always() && inputs.ccache_enabled == 'true'
       uses: ./.github/actions/xahau-actions-cache-save
       with:
-        path: ~/.ccache-main
-        key: ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ inputs.main_branch }}
-        aws-access-key-id: ${{ inputs.aws-access-key-id }}
-        aws-secret-access-key: ${{ inputs.aws-secret-access-key }}
-
-    - name: Save ccache directory for current branch
-      if: always() && inputs.ccache_enabled == 'true' && steps.safe-branch.outputs.name != inputs.main_branch
-      uses: ./.github/actions/xahau-actions-cache-save
-      with:
-        path: ~/.ccache-current
+        path: ~/.ccache
         key: ${{ runner.os }}-ccache-v${{ inputs.cache_version }}-${{ inputs.compiler-id }}-${{ inputs.configuration }}-${{ steps.safe-branch.outputs.name }}
         aws-access-key-id: ${{ inputs.aws-access-key-id }}
         aws-secret-access-key: ${{ inputs.aws-secret-access-key }}