ci: update-flake: handle existing PRs and avoid silent errors

Co-authored-by: Matt Sturgeon <matt@sturgeon.me.uk>

.github/workflows/update-flake.yml

@@ -75,15 +75,32 @@ jobs:
           GH_TOKEN: ${{ steps.generate-token.outputs.token }}
           base_branch: ${{ matrix.branch }}
           body: "This is an automated update triggered by the [workflow run #${{ github.run_id }}](${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }})."  # yamllint disable-line rule:line-length
+          label: "topic: dependencies"
           pr_branch: update_flake_lock_action_${{ matrix.branch }}
           title: "${{ startsWith(matrix.branch, 'release') && format('[{0}] ', matrix.branch) || '' }}flake: update public and dev inputs"  # yamllint disable-line rule:line-length
         run: |
           git switch --create "$pr_branch"
           git push origin "$pr_branch" --force --set-upstream
 
-          gh pr create \
-            --base "$base_branch" \
-            --body "$body" \
-            --label "topic: dependencies" \
-            --title "$title" ||
-            echo "Failed to create PR"
+          pr_count="$(
+            gh api \
+              --method GET \
+              "/repos/$GITHUB_REPOSITORY/pulls" \
+              --field per_page=1 \
+              --raw-field head="$GITHUB_REPOSITORY_OWNER:$pr_branch" \
+              --jq length
+          )"
+
+          if ((pr_count)); then
+            gh pr edit \
+              --body "$body" \
+              --label "$label" \
+              --title "$title"
+
+          else
+            gh pr create \
+              --base "$base_branch" \
+              --body "$body" \
+              --label "$label" \
+              --title "$title"
+          fi