meson: fix chroot build

/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