buildtools: Always reset the build environment

buildtools: Always reset the build environment

In install_library, the Build object's environment was not reset
after an early return, so the extrapython env would be used in
subsequent build steps.
Wrap everything in a try-finally block to make sure the env is reset.

(Almost all of the change is indentation, `git show -w` recommended.)

Signed-off-by: Petr Viktorin <pviktori@redhat.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jelmer Vernooij <jelmer@samba.org>