devel/py-maturin: Fix command name for non-default Python versions

If devel/py-maturin is built for a non-default python flavor (e.g.,
'make FLAVOR=py38'), the port installs bin/maturin-<ver> and does not
install bin/maturin which is normal and expected.

The site-packages/maturin/__init__.py file, however, explicitly defines
the command to use as 'maturin'.

Because of this when using maturin to build other ports, they can fail
with "No such file or directory: 'maturin'". [1] Fix the issue by
aligning the command name to the actual Python flavor.

PR: 272618, 272600 [1]
Reported by: John W. O'Brien [1]
MFH: 2023Q3 (after 1 week)