workflow: free disk space before building

We recently started to hit `ENOSPC` when building the container images
for some reason. Try this `free-disk-space` action to try to fix that.

.github/workflows/build.yml

@@ -40,6 +40,8 @@ jobs:
       matrix:
         releasever: ${{fromJson(needs.query.outputs.matrix)}}
     steps:
+      - name: Free Disk Space (Ubuntu)
+        uses: jlumbroso/free-disk-space@v1.2.0
       - name: Check out repository
         uses: actions/checkout@v3
       - name: Build