gitAndTools.stgit: Fix broken package. (#84198)

The stgit package is installing using make, which leaves the application in a state where it can not properly find the installed package stgit.

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File "/run/current-system/sw/bin/stg", line 35, in <module>
    from stgit.main import main
ImportError: No module named stgit.main
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This is fixed by using buildPythonApplication instead of stdenv.mkDerivation.