[ci] Disable the use of docker on macOS.

The docker setup action is flaky, and the issue tracker shows that
this is a common problem:

https://github.com/docker-practice/actions-setup-docker/issues

Author: ChrisCummins

.github/workflows/ci.yaml

@@ -360,42 +360,6 @@ jobs:
               uses: codecov/codecov-action@v2
 
     test-gcc-env-macos:
-        needs: build-macos
-        runs-on: macos-latest
-        steps:
-            - uses: docker-practice/actions-setup-docker@master
-
-            - name: Confirm docker install
-              run: docker version
-
-            - uses: actions/checkout@v2
-
-            - name: Set up Python
-              uses: actions/setup-python@v2
-              with:
-                  python-version: 3.9
-
-            - name: Download Python wheel
-              uses: actions/download-artifact@v2
-              with:
-                  name: macos-wheel
-
-            - name: Install wheel
-              run: python -m pip install *.whl
-
-            - name: Install runtime dependencies
-              uses: ./.github/actions/install-runtime-dependencies
-
-            - name: Install test dependencies
-              run: python -m pip install -r tests/requirements.txt
-
-            - name: Run the test suite
-              run: make install-test-cov TEST_TARGET="tests/gcc"
-
-            - name: Upload coverage report to Codecov
-              uses: codecov/codecov-action@v2
-
-    test-gcc-env-macos-no-docker:
         needs: build-macos
         runs-on: macos-latest
         steps:
@@ -461,11 +425,6 @@ jobs:
         needs: build-macos
         runs-on: macos-latest
         steps:
-            - uses: docker-practice/actions-setup-docker@master
-
-            - name: Confirm docker install
-              run: docker version
-
             - uses: actions/checkout@v2
 
             - name: Set up Python