workflow: move to github runner again

Author: Redbot

.github/workflows/sync_upstream.yaml

@@ -15,11 +15,12 @@ jobs:
     if: github.repository == 'redguides/eqlib'
     runs-on: ubuntu-latest
     env:
-      REPO_DIR: ${{ runner.temp }}/eqlib
       RG_SYNC_PAT: ${{ secrets.RG_SYNC_PAT }}
     steps:
       - name: Merge macroquest/eqlib (except live, test)
         shell: pwsh
+        env:
+          REPO_DIR: ${{ runner.temp }}/eqlib # runner ctx is invalid in job-level env
         run: |
           $auth = "https://x-access-token:$env:RG_SYNC_PAT@github.com/redguides/eqlib.git"