Fix Python linux distribtest for aarch64_alpine target and enable for presubmit test (grpc#39647)

Following the fix in grpc#39558 to separate the build job for musllinux_1_1_aarch64, looks like the distrib_target step was also not working mutually exclusive with the exclude filters as expected.

The target `python_linux_aarch64_alpine` in the distrib_targets step should also only be part of the `grpc_distribtests_python_arm64` kokoro job but is wrongly getting executed as part of the `grpc_distribtests_python` kokoro job and hence failing on master currently. Hence updated the exclude filters to separate it.

Also, the `python_linux_aarch64_alpine` distrib_target was not running as part of the presubmit tests, which is why this wasn't flagged earlier during the grpc#39558 PR. Given that we want to ensure the future build jobs stay separated and not cause release blockers, I think it would be better to enable this distrib target for presubmits. Have included this change too in this PR.

Closes grpc#39647

COPYBARA_INTEGRATE_REVIEW=grpc#39647 from sreenithi:fix_distribtest_alpine 609ffde
PiperOrigin-RevId: 762584247

Author: sreenithi

tools/internal_ci/linux/grpc_distribtests_python.sh

@@ -68,12 +68,11 @@ cp -r artifacts/* input_artifacts/ || true
 # Run all python linux distribtests
 # We run the distribtests even if some of the artifacts have failed to build, since that gives
 # a better signal about which distribtest are affected by the currently broken artifact builds.
-if [[ "${IS_AARCH64_MUSL}" == "True" ]]; then
-  # We're using alpine as tag in distribtest targets.
-  tools/run_tests/task_runner.py -f distribtest linux python aarch64 alpine -j 12 -x distribtests/sponge_log.xml || FAILED="true"
-else
-  tools/run_tests/task_runner.py -f distribtest linux python ${TASK_RUNNER_EXTRA_FILTERS} -j 12 -x distribtests/sponge_log.xml || FAILED="true"
-fi
+
+# We're using alpine as tag in distribtest targets for musllinux_1_1 artifacts, so exclude filters must use this tag 
+DISTRIB_TASK_RUNNER_EXTRA_FILTERS="${TASK_RUNNER_EXTRA_FILTERS//musllinux_1_1/alpine}"
+
+tools/run_tests/task_runner.py -f distribtest linux python ${DISTRIB_TASK_RUNNER_EXTRA_FILTERS} -j 12 -x distribtests/sponge_log.xml || FAILED="true"
 
 # This step checks if any of the artifacts exceeds a per-file size limit.
 tools/internal_ci/helper_scripts/check_python_artifacts_size.sh

tools/run_tests/artifacts/distribtest_targets.py

@@ -461,7 +461,7 @@ def targets():
         PythonDistribTest(
             "linux", "aarch64", "python39_buster", presubmit=True
         ),
-        PythonDistribTest("linux", "aarch64", "alpine"),
+        PythonDistribTest("linux", "aarch64", "alpine", presubmit=True),
         PythonDistribTest(
             "linux", "x64", "alpine3.18", source=True, presubmit=True
         ),