nixpkgs: tidy up pgjwt package

This makes pgjwt take a dummy 'postgresql' argument, which it does not *need*
in the buildInputs (it is purely a SQL extension with no C code). However, this
argument will be necessary for an upcoming change that will parameterize the
extensions over a particular PostgreSQL version.

It also does some tiny cleanup, setting a null build phase.

Signed-off-by: Austin Seipp <aseipp@pobox.com>