Pinned Ubuntu version for tests & release workflows

- we were using `ubuntu-latest`, which is an alias for 18.04, but there's an
  increasingly likely chance that the default becomes 20.04 soon.
- we don't officially support 20.04 yet, so we want to pin to our
  supported version until we're ready

.github/workflows/release.yml

@@ -5,7 +5,7 @@ on:
       - '*'
 jobs:
   automate:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       RELEASE_TOKEN: ${{ secrets.RELEASE_TOKEN }}
       NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}

.github/workflows/test.yml

@@ -11,7 +11,7 @@ env:
   FORCE_COLOR: 1
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
     strategy:
       matrix:
@@ -59,7 +59,7 @@ jobs:
   ghost-cli:
     name: Ghost-CLI
     if: github.event_name == 'push' || (github.event_name == 'pull_request' && !startsWith(github.head_ref, 'renovate/'))
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
         with: