.github/workflows/build.yaml: switch to ubuntu-latest

.github/workflows/build.yaml

@@ -14,7 +14,7 @@ jobs:
   # Lint changed templates.
   xlint:
     name: Lint templates
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     env:
       PATH: '/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/sbin:/usr/local/bin:/tmp/bin'
@@ -34,7 +34,7 @@ jobs:
   # Build changed packages.
   build:
     name: Build packages
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     if: "!contains(github.event.pull_request.title, '[ci skip]') && !contains(github.event.pull_request.body, '[ci skip]')"
 
     container: