Only run 2and3 tests on platforms that have Python 2 and 3.

With the addition of #203, a missing Python 2 binary will raise an error
instead of assuming that the binary is called `python`. As a result,
this test script now fails on platforms where no Python 2 installation
is available.

.github/workflows/ci.yaml

@@ -42,5 +42,7 @@ jobs:
         run: |
           earthly --ci +test-pypi-install --OS=${{matrix.os}}
       - name: Run 2and3 tests
+        # This test can only be run on platforms that have Python 2 and Python 3 packages.
+        if: ${{contains(["ubuntu:focal", "debian:bullseye", "ubuntu:jammy"], matrix.os)}}
         run: |
           earthly --ci +test-2and3 --OS=${{matrix.os}}