Auto merge of #124405 - RalfJung:miri-core-alloc-tests, r=clubby789

miri core/alloc tests: do not test a 2nd target

check-aux seems to be one of the slowest runners since we started running standard library tests in Miri on it. So maybe it'd be better to reduce test coverage a bit by not doing cross-target testing of core and alloc? I don't recall finding target-specific issues in these libraries ever (and we still have the extra test coverage via our [out-of-tree nightly tests](https://github.com/rust-lang/miri-test-libstd)). This gives us more buffer to deal with the fact that the number of tests we run will only grow over time.

Cc `@rust-lang/miri` `@rust-lang/infra`

Author: bors