fix(toolchain) Override coverage rc

Author: ewianda

python/private/stage2_bootstrap_template.py

@@ -309,29 +309,28 @@ def _maybe_collect_coverage(enable):
 relative_files = True
 source =
 \t{source}
+omit = 
+            # Pipes can't be read back later, which can cause coverage to
+            # throw an error when trying to get its source code.
+           /dev/fd/* 
+            # The mechanism for finding third-party packages in coverage-py
+            # only works for installed packages, not for runfiles. e.g:
+            #'$HOME/.local/lib/python3.10/site-packages',
+            # '/usr/lib/python',
+            # '/usr/lib/python3.10/site-packages',
+            # '/usr/local/lib/python3.10/dist-packages'
+            # see https://github.com/nedbat/coveragepy/blob/bfb0c708fdd8182b2a9f0fc403596693ef65e475/coverage/inorout.py#L153-L164
+           */external/*
 """
         )
     try:
         cov = coverage.Coverage(
-            config_file=rcfile_name,
+            config_file=os.environ.get("COVERAGE_RCFILE", rcfile_name),
             branch=True,
             # NOTE: The messages arg controls what coverage prints to stdout/stderr,
             # which can interfere with the Bazel coverage command. Enabling message
             # output is only useful for debugging coverage support.
             messages=is_verbose_coverage(),
-            omit=[
-                # Pipes can't be read back later, which can cause coverage to
-                # throw an error when trying to get its source code.
-                "/dev/fd/*",
-                # The mechanism for finding third-party packages in coverage-py
-                # only works for installed packages, not for runfiles. e.g:
-                #'$HOME/.local/lib/python3.10/site-packages',
-                # '/usr/lib/python',
-                # '/usr/lib/python3.10/site-packages',
-                # '/usr/local/lib/python3.10/dist-packages'
-                # see https://github.com/nedbat/coveragepy/blob/bfb0c708fdd8182b2a9f0fc403596693ef65e475/coverage/inorout.py#L153-L164
-                "*/external/*",
-            ],
         )
         cov.start()
         try: