libselinux: Fix cross-compilation with Python 3

[samueldr]

Motivation for this change

Fixes cross compilation, after #75668.

Without this, the python used somehow is the target python, and not the build python. Not sure what changed between python3 and python2 that made this a new issue.

With #77525 and this, Mobile NixOS stage-1 builds correctly on the tip of staging.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[samueldr]

Ah, @lopsided98 explained it, I believe, in #75668 already. See #49526.