Rollup merge of #128038 - compiler-errors:inc-fat, r=oli-obk

Don't output incremental test artifacts into working directory

Currently tests can ICE when the test spits out `inc-fat` incremental artifacts directly into the top of the git checkout, and then the compiler version changes, and it reads nonsense incremental artifacts on a subsequent test run.

r? `@oli-obk`

cc `@Oneirical,` I think you added this -- I think the right flag to add when porting `-Cincremental` run-make tests is to use `//@ incremental` rather than manually specifying the `-Cincremental` rustflag.