nx-libs: minor fixes

* Add an alias with a deprecation warning for `nxproxy` to avoid an
  immediate breaking change.

* Use the default shell used in the build environment (`stdenv.shell`)
  for patching. This shell is in the environment and thus used to patch
  scripts using `patchShebangs`. The shell is referenced as `stdenv.shell`
  in Makefiles to patch the remaining occurrences of `/bin/bash` in the
  build environment.