https://github.com/NixOS/nixpkgs/blob/master/doc/builders/special/makesetuphook.section.md

broken by "qt5.wrapQtAppsHook: move libs to depsTargetTargetPropagated"
qtbase propagates a lot of stuff

frescobaldi> msgcat -o - frescobaldi/cs.po userguide/cs.po | msgfmt -o ../frescobaldi_app/i18n/cs.mo -
frescobaldi> sh: line 1: msgcat: command not found
frescobaldi> sh: line 1: msgfmt: command not found

This commit fixes builds of `pkgsCross.*.qtbase` by adding the
buildPlatform compiler to depsBuildBuild in qtbase.nix and
qtModule.nix.  The `qtbase` build machinery expects to find it in
the $PATH in unprefixed form.

Without this, two different copies of qtbase to wind up in the
buildInputs for cross compiled builds, leading to this sort of
breakage for PyQt:

```
Error: detected mismatched Qt dependencies:
     /nix/store/8g7xc01pj6y569qfyf5ccfbs5dr87wwq-qtbase-aarch64-unknown-linux-gnu-5.15.9-dev
     /nix/store/w19n5gzlx4fv63a3dn5965s661nhakv9-qtbase-5.15.9-dev
```

I was able to get PyQt5 to cross compile with this commit.