ci: Free up disk space asynchronously

Free-ing up disk space can take up to 8 minutes so let's make sure
we do it asynchronously since we don't need the free space immediately.

.github/workflows/ci.yml

@@ -125,10 +125,14 @@ jobs:
     - uses: actions/checkout@v3
     - uses: ./
 
+    # Freeing up disk space with rm -rf can take multiple minutes. Since we don't need the extra free space
+    # immediately, we remove the files in the background. However, we first move them to a different location so that
+    # nothing tries to use anything in these directories anymore while we're busy deleting them.
     - name: Free disk space
       run: |
-        sudo rm -rf /usr/local
-        sudo rm -rf /opt/hostedtoolcache
+        sudo mv /usr/local /usr/local.trash
+        sudo mv /opt/hostedtoolcache /opt/hostedtoolcache.trash
+        sudo systemd-run rm -rf /usr/local.trash /opt/hostedtoolcache.trash
 
     - name: Install
       run: |