math/Imath: ensure PYTHON build with /usr/bin/python[3] links

When /usr/bin/python[3] symlinks are present (which is nonstandard), the
FindPython logic inside cmake will grab it over $PREFIX/bin/python[3],
but still identify (by default) /usr/local/lib/... as the sitelib path,
but goof up when calculating the relative path in case of /usr/bin vs
/usr/local/bin mismatch.

Fix: Provide ${PYTHON_CMD} through the cmake Python_EXECUTABLE
variable to avoid any ambiguities and pin the right executable.

I believe it should be safe to NOT bump PORTREVISION,
but I will fetch far and err on the safe side and bump it, because that
is easier than researching all possible paths how and where
Python paths might find their way into the resulting package.

The detailed report by Seneca helped identify and solve this issue, so:
Thanks to: Seneca Cunningham <seneca@vybenetworks.com>

PR: 263341
Reported by: D'Arcy J.M. Cain <darcy@druid.net>
Reported by: Seneca Cunningham <seneca@vybenetworks.com>
MFH: 2022Q3