Fix/baseline cache refresh 2 (#493)

* Fix baseline cache invalidation

* Fix PR detection for baseline cache

Author: cvanelteren

.github/workflows/build-ultraplot.yml

@@ -55,7 +55,7 @@ jobs:
     name: Compare baseline Python ${{ inputs.python-version }} with MPL ${{ inputs.matplotlib-version }}
     runs-on: ubuntu-latest
     env:
-      IS_PR: ${{ github.event.pull_request }}
+      IS_PR: ${{ github.event_name == 'pull_request' }}
     defaults:
       run:
         shell: bash -el {0}