waf: fix cross-compiling by not overriding python in all-packages.nix

[Artturin]

instead use python3 explicitly in the expression

the issue is caused by #49526

Motivation for this change

Supersedes #107000

Things done

Tested cross with nix build ".#pkgsCross.aarch64-multiplatform.waf"

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages 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/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[r-burns]

LGTM. Would setting strictDeps help avoid regressions?

[Artturin]

LGTM. Would setting strictDeps help avoid regressions?

yes i think so.

[Artturin]

@ofborg eval

[Mindavi]

Diff LGTM. Consider cross-building one of the packages that depend on this as well (they will of course run the compiler on the build platform, but it's a good sanity check), e.g. by applying this patch: #140292