meson: add back /usr/bin/env to the python-cmd. Fixes #237

From nirbheek: "The shebang parsing goes like this: everything before the first space is the interpreter, everything after that is a single argument to that interpreter. So in meson, if the interpreter in the shebang is env, we ignore it and slurp the rest as the actual interpreter and parse it with shell syntax to handle spaces correctly.

IIRC the py.exe python launcher on windows also knows that it should ignore /usr/bin/env and look at the rest to find the actual interpreter you want to use."