GitHub action runner OS not set

[wsdewitt]

The matrix for this build/test workflow specifies ubuntu and macos, but the first line hard codes the runner to be ubuntu.

gctree/.github/workflows/build-and-test.yml

Lines 12 to 16 in 37792e0

	runs-on: ubuntu-latest
	strategy:
	matrix:
	os: [ubuntu-latest, macos-latest]
	python-version: [3.7, 3.8, 3.9]

I think we want to replace to something like:

    strategy:
      matrix:
        os: [ubuntu-latest, macos-latest]
        python-version: [3.7, 3.8, 3.9]
    runs-on: ${{ matrix.os }}

Or if running only on ubuntu was intended, we can remove macos from the matrix.