ci: update to ubuntu-latest

We were doing that everywhere already except for the 'rustfmt' runner.

Author: BurntSushi

.github/workflows/ci.yml

@@ -177,7 +177,7 @@ jobs:
 
   rustfmt:
     name: rustfmt
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - name: Checkout repository
       uses: actions/checkout@v3