Override sys.prefix only if it hasn't been changed

The purpose of the sitecustomize here is to make setuptools install
stuff to our prefix instead of the global prefix, and to appear enough
like a virtual environment that platform-specific patches to sysconfig
don't interfere with that effort.

It isn't inconceivable that there may be subprocesses involved in the
build, and we want any installation happening in those subprocesses to
also get the redirect, and that currently works as expected as long as
the sitecustomize directory remains on PYTHONPATH somewhere.

However, if some subprocess is also modifying sys.path, as would be the
case if a subprocess was using a new virtual environment, we don't want
to override those changes. Since we know what sys.prefix was when colcon
was invoked, we can just check to see if it changed.

colcon_core/task/python/build.py

@@ -70,6 +70,7 @@ async def build(self, *, additional_hooks=None): # noqa: D102
  Path(__file__).parent / 'template' / 'sitecustomize.py.em',
  prefix_override / 'sitecustomize.py',
  {
+ 'current_prefix': sys.prefix,
  'site_prefix': args.install_base,
  })
 

colcon_core/task/python/template/sitecustomize.py.em

@@ -1,4 +1,4 @@
 import sys
-if sys.prefix == sys.base_prefix:
+if sys.prefix == @repr(current_prefix):
  sys.real_prefix = sys.prefix
  sys.prefix = sys.exec_prefix = @repr(site_prefix)