Stop update-api-spec from editing open PRs (#80)

The behavior of `peter-evans/create-pull-request` is to check if either
the base or target branches have changed and either keep creating new
PRs or update an open PR (default), force-pushing and editing its
description.

Prevent the action from running on open PRs entirely, because API spec
updates will often require manual changes to be pushed, and the workflow
shouldn't force-push over them.

Author: gabrielfeo

.github/workflows/update-api-spec.yml

@@ -33,14 +33,24 @@ jobs:
             echo "NEW_VERSION=$(cat new.txt)" >> $GITHUB_ENV
           fi
           rm new.txt || true
+      - name: Check for existing PR
+        if: ${{ env.NEW_VERSION }}
+        run: |
+          set -u
+          update_branch="feature/api-spec-$NEW_VERSION"
+          echo "UPDATE_BRANCH=$update_branch" >> $GITHUB_ENV
+          if git ls-remote --exit-code --heads origin "$update_branch"; then
+            echo "EXISTING_PR=true" >> $GITHUB_ENV
+          fi
       - name: 'Create PR'
-        if: ${{ inputs.dry_run != true && env.NEW_VERSION }}
+        if: ${{ inputs.dry_run != true && env.NEW_VERSION && !env.EXISTING_PR }}
         uses: peter-evans/create-pull-request@v5
         with:
-          branch: "feature/api-spec-${{ env.NEW_VERSION }}"
+          branch: "${{ env.UPDATE_BRANCH }}"
           commit-message: "Bump GE API spec version to ${{ env.NEW_VERSION }}"
           title: "Bump GE API spec version to ${{ env.NEW_VERSION }}"
           body: "https://docs.gradle.com/enterprise/api-manual/#release_history"
           author: "github-actions <github-actions@github.com>"
           committer: "github-actions <github-actions@github.com>"
           add-paths: "gradle.properties"
+          delete-branch: true