chore: only test Mathlib on PRs to main (#977)

.github/workflows/test_mathlib.yml

@@ -20,7 +20,7 @@ jobs:
  sourceRunId: ${{ github.event.workflow_run.id }}
 
  - name: Checkout mathlib4 repository
- if: steps.workflow-info.outputs.pullRequestNumber != ''
+ if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.workflow-info.outputs.targetBranch == 'main'
  uses: actions/checkout@v4
  with:
  repository: leanprover-community/mathlib4
@@ -29,14 +29,15 @@ jobs:
  fetch-depth: 0
 
  - name: install elan
+ if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.workflow-info.outputs.targetBranch == 'main'
  run: |
  set -o pipefail
  curl -sSfL https://github.com/leanprover/elan/releases/download/v3.0.0/elan-x86_64-unknown-linux-gnu.tar.gz | tar xz
  ./elan-init -y --default-toolchain none
  echo "$HOME/.elan/bin" >> "${GITHUB_PATH}"
 
  - name: Retrieve PR information
- if: steps.workflow-info.outputs.pullRequestNumber != ''
+ if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.workflow-info.outputs.targetBranch == 'main'
  id: pr-info
  uses: actions/github-script@v6
  env:
@@ -53,7 +54,7 @@ jobs:
  core.exportVariable('HEAD_BRANCH', pr.head.ref);
 
  - name: Check if tag exists
- if: steps.workflow-info.outputs.pullRequestNumber != ''
+ if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.workflow-info.outputs.targetBranch == 'main'
  id: check_mathlib_tag
  env:
  PR_NUMBER: ${{ steps.workflow-info.outputs.pullRequestNumber }}
@@ -90,7 +91,7 @@ jobs:
  fi
 
  - name: Push changes
- if: steps.workflow-info.outputs.pullRequestNumber != ''
+ if: steps.workflow-info.outputs.pullRequestNumber != '' && steps.workflow-info.outputs.targetBranch == 'main'
  env:
  PR_NUMBER: ${{ steps.workflow-info.outputs.pullRequestNumber }}
  run: |