In configured builds other tools tries to call meson by directly
passing meson tools directly to python. Since they were shell scripts
due wrapping this failed
/usr/bin/env is not available in chroot builds. Invoke the python3
interpreter directly instead of trying to let env do it (which fails).
Fixes this build error:
$ nix-build -A meson
...
/nix/store/HASH-stdenv/setup: ./install_meson.py: /usr/bin/env: bad interpreter: No such file or directory
builder for ‘/nix/store/HASH-meson-0.26.0.drv’ failed with exit code 126
