multipath-tools: ensure gzip does not capture timestamp

gzip is originally called as 'gzip -9 -c'

This is a port of
https://github.com/NixOS/nixpkgs/pull/2281/commits/a8e7ddd1dfaae4b3ce97fdefe4614a3e31c04501

Note that it does not seem to make a difference to `nix-build --check`.