Let pytest-cov create the xml directly (#33619)

`coverage` sometimes failed to combine, even if there were multiple reports.

.github/workflows/unit_tests.yaml

@@ -89,8 +89,6 @@ jobs:
           UNIT_TEST_COVERAGE: ${{ (matrix.python-version == '3.10') }}
       run: |
           share/spack/qa/run-unit-tests
-          coverage combine -a
-          coverage xml
     - uses: codecov/codecov-action@d9f34f8cd5cb3b3eb79b3e4b5dae3a16df499a70
       with:
         flags: unittests,linux,${{ matrix.concretizer }}
@@ -182,8 +180,6 @@ jobs:
           SPACK_TEST_SOLVER: clingo
       run: |
           share/spack/qa/run-unit-tests
-          coverage combine -a
-          coverage xml
     - uses: codecov/codecov-action@d9f34f8cd5cb3b3eb79b3e4b5dae3a16df499a70 # @v2.1.0
       with:
         flags: unittests,linux,clingo
@@ -218,9 +214,7 @@ jobs:
         $(which spack) bootstrap disable spack-install
         $(which spack) solve zlib
         common_args=(--dist loadfile --tx '4*popen//python=./bin/spack-tmpconfig python -u ./bin/spack python' -x)
-        $(which spack) unit-test --cov --cov-config=pyproject.toml "${common_args[@]}"
-        coverage combine -a
-        coverage xml
+        $(which spack) unit-test --cov --cov-config=pyproject.toml --cov-report=xml:coverage.xml "${common_args[@]}"
     - uses: codecov/codecov-action@d9f34f8cd5cb3b3eb79b3e4b5dae3a16df499a70
       with:
         flags: unittests,macos