ci: set origin/HEAD before running release.sh

actions/checkout does not create refs/remotes/origin/HEAD, so
release.sh's default_branch() resolved to an empty string and the first
Cut Release run failed on an empty pathspec.

Author: eddietejeda

.github/workflows/cut-release.yml

@@ -72,6 +72,12 @@ jobs:
           git config user.name "$slug"
           git config user.email "${id}+${slug}@users.noreply.github.com"
 
+      # release.sh resolves the default branch from origin/HEAD, which
+      # actions/checkout never sets; without this the script sees an empty
+      # branch name and git aborts on an empty pathspec.
+      - name: Point origin/HEAD at the default branch
+        run: git remote set-head origin --auto
+
       - name: Prepare release
         env:
           GH_TOKEN: ${{ steps.app-token.outputs.token }}
@@ -125,6 +131,12 @@ jobs:
           git config user.name "$slug"
           git config user.email "${id}+${slug}@users.noreply.github.com"
 
+      # release.sh resolves the default branch from origin/HEAD, which
+      # actions/checkout never sets; without this the script sees an empty
+      # branch name and git aborts on an empty pathspec.
+      - name: Point origin/HEAD at the default branch
+        run: git remote set-head origin --auto
+
       - name: Tag and push
         env:
           GH_TOKEN: ${{ steps.app-token.outputs.token }}