fix(ci): select stable v1 backend tag

Co-authored-by: Miles Cranmer <miles.cranmer@gmail.com>

Author: MilesCranmerBot

.github/workflows/update_backend.yml

@@ -18,13 +18,18 @@ jobs:
           python -m pip install --upgrade pip
           pip install tomlkit
 
-      - name: "Get SymbolicRegression.jl latest version"
+      - name: "Get SymbolicRegression.jl latest stable v1 version"
         id: get-latest
         run: |
           cd $(mktemp -d)
           git clone https://github.com/MilesCranmer/SymbolicRegression.jl
           cd SymbolicRegression.jl
-          echo "version=$(git describe --tags --match='v*' --abbrev=0 | sed 's/^v//')" >> $GITHUB_OUTPUT
+          latest_tag=$(git tag --list 'v1.*' | grep -E '^v1\.[0-9]+\.[0-9]+$' | sort -V | tail -n1)
+          if [ -z "$latest_tag" ]; then
+            echo "Failed to find a stable v1.*.* tag in SymbolicRegression.jl" >&2
+            exit 1
+          fi
+          echo "version=${latest_tag#v}" >> $GITHUB_OUTPUT
 
       - name: "Update SymbolicRegression.jl version in PySR"
         run: |
@@ -40,6 +45,7 @@ jobs:
       - name: "Create PR if necessary"
         uses: peter-evans/create-pull-request@v7
         with:
+          base: release-v1
           title: "Automated update to backend: v${{ steps.get-latest.outputs.version }}"
           body: |
             This PR was automatically generated by the GitHub Action `.github/workflows/update-backend.yml`