ci: Cache NX cache in GHA

Author: mydea

.github/workflows/build.yml

@@ -174,6 +174,11 @@ jobs:
         with:
           path: ${{ env.CACHED_BUILD_PATHS }}
           key: ${{ env.BUILD_CACHE_KEY }}
+      - name: NX cache
+        uses: actions/cache@v3
+        with:
+          path: node_modules/.cache/nx
+          key: ${{ runner.os }}-nx
       - name: Build packages
         # Under normal circumstances, using the git SHA as a cache key, there shouldn't ever be a cache hit on the built
         # packages, and so `yarn build` should always run. This `if` check is therefore only there for testing CI issues