bpo-44062: Fix PYTHON_FOR_BUILD interpreter lookup for cross-compile …

[vfazio]

…builds

Previously, when running configure in a cross-compile scenario, the
PYTHON_FOR_BUILD interpreter could be set to python even if no python
is availble in PATH.

Now, configure will properly set PYTHON_FOR_BUILD based on what is
available in PATH or fail.

https://bugs.python.org/issue44062

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[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[vfazio]

Closing since #90039 (bpo-45881) was addressed in #29835

#88228 should be closed to reflect this PR being closed.