Don't pass COV_CORE_SOURCE to python build subprocess

It appears that when this varible is set, pytest-cov will import modules
during the interpreter initialization before the sitecustomize hook
runs. Among the modules imported is sysconfig, which caches its global
state before we're able to modify it, thus rendering the entire hook
unusable.

The documentation for the 'site' module has more details here, but I
wasn't able to find a more aggressive way to prioritize our hook over
others. For now, we should suppress COV_CORE_SOURCE to unblock new tests
in colcon itself.

colcon_core/task/python/build.py

@@ -76,6 +76,8 @@ async def build(self, *, additional_hooks=None):  # noqa: D102
         env = dict(env)
         env['PYTHONPATH'] = str(prefix_override) + os.pathsep + \
             python_lib + os.pathsep + env.get('PYTHONPATH', '')
+        # coverage capture interferes with sitecustomize
+        env.pop('COV_CORE_SOURCE', None)
 
         # determine if setuptools specific commands are available
         available_commands = await self._get_available_commands(