CI: update gap-actions/build-pkg to v3

Author: fingolfin

.github/workflows/CI.yml

@@ -37,7 +37,7 @@ jobs:
       - uses: gap-actions/setup-gap@v3
         with:
           gap-version: ${{ matrix.gap-version }}
-      - uses: gap-actions/build-pkg@v2
+      - uses: gap-actions/build-pkg@v3
       - uses: gap-actions/run-pkg-tests@v4
       - uses: gap-actions/run-pkg-tests@v4
         with: