fix(ci): use env var for API_BASE in RC release step

The Python heredoc couldn't access shell-local API variable.
Move it to step-level env so os.environ sees it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.mokogitea/workflows/pr-rc-release.yml

@@ -115,20 +115,19 @@ jobs:
           PR_URL: ${{ github.event.pull_request.html_url }}
           PR_NUMBER: ${{ github.event.pull_request.number }}
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          API_BASE: https://${{ env.REGISTRY }}/api/v1/repos/${{ github.repository }}
         run: |
-          API="https://${REGISTRY}/api/v1/repos/${{ github.repository }}"
-
           # Delete existing RC release/tag if present
           curl -s -X DELETE -H "Authorization: token ${GITEA_TOKEN}" \
-            "${API}/releases/tags/${RC_TAG}" 2>/dev/null || true
+            "${API_BASE}/releases/tags/${RC_TAG}" 2>/dev/null || true
           curl -s -X DELETE -H "Authorization: token ${GITEA_TOKEN}" \
-            "${API}/tags/${RC_TAG}" 2>/dev/null || true
+            "${API_BASE}/tags/${RC_TAG}" 2>/dev/null || true
 
           # Create prerelease
           python3 << PYEOF
           import json, os, urllib.request
 
-          api = os.environ["API"]
+          api = os.environ["API_BASE"]
           token = os.environ["GITEA_TOKEN"]
           payload = json.dumps({
               "tag_name": os.environ["RC_TAG"],