Fix promoteToPublic.finalize github:pr step

The final github:pr step in the promoteToPublic.finalize workflow was
incorrectly trying to create a PR from the current branch to itself,
causing 'must be on a branch named differently' errors.

Root cause: Variable internal_branch was set to the current branch name
'promote/release-{{version}}-update-main' instead of the target branch 'main'.

This step is intended to create a PR to merge the updated release images
into the main branch, so the --base should be 'main', not the current branch.

Fixes the systematic release pipeline failures in promote branches.

Author: DaedalusG

release.yaml

@@ -278,13 +278,11 @@ promoteToPublic:
       - name: "github:pr"
         cmd: |
           set -eu
-          internal_branch="promote/release-{{version}}-update-main"
-          # we need to fetch from origin just in case this branch doesn't exist locally, so that the PR can find the base
-          git fetch origin "${internal_branch}"
+          # Create PR to merge release updates into main branch
           gh pr create \
             --fill \
             --draft \
-            --base "$internal_branch" \
+            --base main \
             --title "Update main: build {{version}}" \
             --body "Test plan: automated release PR, CI will perform additional checks"
           echo "🚢 Please check the associated CI build to ensure the process completed".