tests/crashes/124436.rs is creating a `mir_dump` directory in the repo root, not the build directory

[saethlin]

#124775 added some tests, and one of them is creating a mir_dump directory in the repo root, where it really shouldn't. Surely we have ways of dumping MIR to other directories for the mir-opt tests, right?

@matthiaskrgr

[compiler-errors]

Yeah this has been annoying me recently. I put up #125032.