Run ambiguity test inside a clean process

[tkf]

fixes #28804

[tkf]

I was previously doing detect_ambiguities(LinearAlgebra, Base; ...) by accident but I suppose I didn't need Base there?

[ararslan]

Looks like 32-bit Windows is hitting a GC bug:

      From worker 6:	
      From worker 6:	Please submit a bug report with steps to reproduce this fault, and any error messages that follow (in their entirety). Thanks.
      From worker 6:	Exception: EXCEPTION_ACCESS_VIOLATION at 0x682b1f5a -- gc_try_setmark at /cygdrive/c/projects/julia/src\gc.c:1428 [inlined]
      From worker 6:	gc_mark_scan_obj8 at /cygdrive/c/projects/julia/src\gc.c:1554 [inlined]
      From worker 6:	gc_mark_loop at /cygdrive/c/projects/julia/src\gc.c:1818
      From worker 6:	in expression starting at C:\projects\julia\julia-e47bb0e211\share\julia\test\sorting.jl:35
      From worker 6:	gc_try_setmark at /cygdrive/c/projects/julia/src\gc.c:1425 [inlined]
      From worker 6:	gc_mark_scan_obj8 at /cygdrive/c/projects/julia/src\gc.c:1554 [inlined]
      From worker 6:	gc_mark_loop at /cygdrive/c/projects/julia/src\gc.c:1818

[Keno]

Let's watch for failures like that after merging. I'm hoping one of my recent gc root PRs may have fixed it, but wasn't picked up in this CI run yet.