Fix location of compiler on linux in shiboken2

[hmaarrfk]

Closes https://github.com/conda-forge/pyside2-feedstock/issues/130

Enables cross compilation on linux. I was able to build Python 3.9 on linux using my own channel in https://github.com/conda-forge/pyside2-feedstock/pull/128

CC: @looooo

Checklist

• [ ] Used a personal fork of the feedstock to propose changes
• [ ] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [ ] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[hmaarrfk]

Given I"ve communicated with upstream on this fact. I think this is safe to merge.