.github/workflows: enable fs-verity on /

...instead of using an extra loopback partition.

Suggested by Colin in containers/composefs#389.

.github/workflows/rust.yml

@@ -15,18 +15,8 @@ jobs:
     runs-on: ubuntu-24.04
 
     steps:
-    - name: Create and mount an fs-verity-enabled filesystem
-      run: |
-        sudo truncate --size 4GiB /verity.fs
-        sudo mkfs.ext4 -O verity /verity.fs
-        sudo mkdir /verity
-        sudo mount -o loop /verity.fs /verity
-        sudo chmod 1777 /verity
-
-        mkdir /verity/worker
-        mkdir -p ~/.var
-        ln -sf /verity/worker ~/.var/tmp
-
+    - name: Enable fs-verity on /
+      run: sudo tune2fs -O verity $(findmnt -vno SOURCE /)
     - uses: actions/checkout@v4
     - name: Build
       run: cargo build --verbose