spack-python: remove superfluous /usr/bin/env (#44724)

Not sure why I had this here, as `/bin/sh` will find the first spack in `PATH` just like
`env`.

- [x] remove `/usr/bin/env` and avoid an extra process launch.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

bin/spack-python

@@ -22,4 +22,4 @@
 #
 # This is compatible across platforms.
 #
-exec /usr/bin/env spack python "$@"
+exec spack python "$@"