Merge pull request #125 from scipp/remove-bad-head_ref

Build/tests must not checkout `github.head_ref`

Author: SimonHeybrock

.github/workflows/pr_and_main.yml

@@ -39,7 +39,6 @@ jobs:
     steps:
       - uses: actions/checkout@v2
         with:
-          ref: ${{ github.head_ref }}
           fetch-depth: 0  # history required so cmake can determine version
       - uses: conda-incubator/setup-miniconda@v2
       - run: conda install --channel conda-forge --yes conda-build mamba boa