Make build-cpython-ghuser-components work on macOS

[gonzalocasas]

Run the componentizer as a direct subprocess (sys.executable) instead of through a shell, and on macOS force the Mono runtime and add the Homebrew library dirs to DYLD_LIBRARY_PATH.

The componentizer loads GH_IO.dll via pythonnet, so no Rhino/Windows is required — only a .NET runtime. On macOS that's Mono, which needs the native libgdiplus to embed component icons. Two things broke this off Windows:

• The Mono that pythonnet embeds doesn't search the Homebrew prefix the way the mono CLI does, so libgdiplus wasn't found. Setting DYLD_LIBRARY_PATH to $(brew --prefix)/lib (plus the standard /opt/homebrew and /usr/local prefixes) fixes resolution.
• macOS SIP strips DYLD_* when a command crosses the protected /bin/sh, which is what invoke's ctx.run spawns. Calling the interpreter directly via subprocess.run keeps the variable alive.

Windows behaviour is unchanged (env returned untouched).

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[chenkasirer]

AMAZING! LGTM!