squash! Don't use the Nix flake

.github/workflows/merge-upstream.yml

@@ -42,17 +42,17 @@ jobs:
         git config --add user.email '6270544+github-actions[bot]@users.noreply.github.com'
 
     - name: Retrieve the merge base
-      id: merge-base
       run: |
         rev=$(git merge-base ${{ steps.upstream.outputs.ref }} ${{ steps.start.outputs.rev }})
-        echo "rev=$rev" >> "$GITHUB_OUTPUT"
+        git switch -C merge-base "$rev"
+        git push --force origin HEAD
 
     - name: Filter commits
       run: |
         tmp="recipes-$(date +%s)"
         git switch -c "$tmp" ${{ steps.upstream.outputs.ref }}
         nix run nixpkgs#git-filter-repo -- \
-          --path recipes/ --refs "${{ steps.merge-base.outputs.rev }}..HEAD" \
+          --path recipes/ --refs "merge-base..HEAD" \
           --commit-callback '
             commit.committer_name = commit.author_name
             commit.committer_email = commit.author_email
@@ -68,7 +68,7 @@ jobs:
     - name: Create a pull request
       uses: peter-evans/create-pull-request@v5
       with:
-        base: ${{ steps.merge-base.outputs.rev }}
+        base: merge-base
         token: ${{ secrets.PAT_FOR_PR }}
         title: 'Merge the upstream recipe commits'
         branch: create-pull-request/merge-recipes