perf(ci): cache XanMod kernel source between runs

Adds actions/cache on kernel/src keyed by branch + year-week,
matching the pattern already used in xanmod-unified-kernel.
Eliminates a ~200-400 MB GitLab clone on every CI run; cache
restores in ~10-30s instead.

Author: Interested-Deving-1896

.github/workflows/build.yml

@@ -1,4 +1,3 @@
-```yaml
 name: Build
 
 on:
@@ -77,6 +76,20 @@ jobs:
             libncurses-dev dwarves pahole curl git jq \
             shellcheck
 
+      - name: Compute kernel cache key
+        run: echo "YEAR_WEEK=$(date -u '+%Y-W%V')" >> "$GITHUB_ENV"
+
+      - name: Restore kernel source cache
+        id: cache-kernel
+        uses: actions/cache@v4
+        with:
+          path: kernel/src
+          # Keyed on branch + year-week so the cache refreshes weekly
+          # to pick up new upstream commits.
+          key: xanmod-kernel-${{ matrix.branch }}-${{ env.YEAR_WEEK }}
+          restore-keys: |
+            xanmod-kernel-${{ matrix.branch }}-
+
       - name: Fetch XanMod source
         run: bash kernel/fetch.sh ${{ matrix.branch }}
         env:
@@ -161,4 +174,3 @@ jobs:
           name: "LiqXanMod build ${{ github.run_number }}"
           files: artifacts/**/*
           generate_release_notes: true
-```