rust cross-compile uses wrong compiler for subprojects

[zmike]

Describe the bug
Trying to cross-compile i686 (probably any cross-compile) mesa/nvk is broken because the internal subprojects are all built with the system compiler instead of the specified i686 toolchain.

To Reproduce
Bulid mesa for a non-native arch with -Dvulkan-drivers=nouveau

Expected behavior
I expected to be able to build the project

system parameters

• Is this a cross build or just a plain native build (for the same computer)?
  • i686 from x86_64
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.)
  • fedora 41
• what Python version are you using e.g. 3.8.0
  • 3.13.1
• what meson --version
  • 1.5.1
• what ninja --version if it's a Ninja build
  • 1.12.1