stdenv.targetPlatform really shouldn't be used by software that
doesn't generate or manipulate binaries. I reviewed all uses of
targetPlatform outside of pkgs/development/compilers and pkgs/stdenv
and replaced those which weren't involved in something which fits
these criteria.
https://github.com/NixOS/nixpkgs/pull/207804 removed git from
nativeBuildInputs for buildRustPackage, which is apparently what allowed
the build.rs to work here. Bring it back for this single package.
This package includes the CLI binaries for mullvad, but does not attempt
to build the GUI. This allows it to be (more) cross-platform than the
mullvad-vpn package, which depends on a tool that is unavailable for
e.g. ARM platforms.