PYTHONBIN for cross-compilation to different LOCALBASE

[Taylor R Campbell <campbell+netbsd-tech-pkg%mumble.net@localhost>]

The variable PYTHONBIN serves two roles in a package's makefile:

1. Some makefile rules, such as the do-build rule to run setup.py,
   will execute it at compile-time.

2. Its value is baked into some packages like in #! lines for
   execution at run-time.

With the TOOLBASE/LOCALBASE split I proposed earlier
(https://mail-index.netbsd.org/tech-pkg/2023/06/11/msg027658.html),
it will become necessary to distinguish these two roles.

I'd like to avoid the somewhat painful TOOL_DEPENDS vs BUILD_DEPENDS
mistake I made of inventing a new name for what is far and away the
most common use.  So which of these two roles for PYTHONBIN is more
common?

I count about four hundred references in pkgsrc, which is a bit much
to eyeball manually for audit, since it's not trivial to tease apart
which one is which at a glance.