build(ci): fix edge publishing on main (#1756)

In some cases, publishing from main to edge fails:
https://github.com/canonical/charmcraft/actions/runs/10062446056/job/27814976023

.github/workflows/tests.yaml

@@ -121,6 +121,9 @@ jobs:
             echo "branch=merge" >> "$GITHUB_OUTPUT"
           else
             branch=$(echo ${{ github.base_ref }} | sed -e 's|feature/\(.*\)|\1|')
+            if [ -z "${branch}" ]; then
+              branch=$(echo ${{ github.ref_name }} | sed -e 's|/|_|')
+            fi
             echo "branch=$branch" >> "$GITHUB_OUTPUT"
           fi
       - name: Publish feature branch to edge/${{ steps.vars.outputs.branch }}