Automatically increment the default version number

In theory, we could use GitHub REST API to query the most recent
release version of the mold linker, but the API fails too often,
so we want to write down the latest release version to our config
file.

.github/workflows/update-version.yml

@@ -0,0 +1,38 @@
+name: Update Default Mold Version
+
+on:
+  schedule:
+    - cron: '0 0 * * *'
+
+jobs:
+  check-update-mold-version:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Update action.yml if needed
+        run: |
+          set -x
+          curl -s https://api.github.com/repos/rui314/mold/releases/latest > info
+          RELEASE_TAG=$(jq -r .tag_name info | sed 's/^v//')
+          RELEASE_DATE=(jq -r .published_at info)
+          CURRENT_TAG=$(grep default: action.yml | head -1 | cut -d "'" -f 2)
+
+          echo "$RELEASE_TAG" | grep -Eq '^[0-9]+\.[0-9]+\.[0-9]+$'
+
+          if [ $CURRENT_TAG = $RELEASE_TAG ]; then
+            echo "action.yml is already up to date"
+          elif [ $(date +%s) -lt $(date -d "$RELEASE_DATE + 7 days" +%s) ]; then
+            echo "still in grace period"
+          else
+            sed -i "s/default: '.*'/default: '$RELEASE_TAG'/" action.yml
+            git config user.name "GitHub Actions"
+            git config user.email "[email protected]"
+            git commit -am "Update default version to $RELEASE_TAG"
+            git push
+            git tag -f v1
+            git push -f --tags
+          fi