Auto merge of #6966 - ehuss:reamp-path-prefix-hash, r=@alexcrichton

Auto merge of #6966 - ehuss:reamp-path-prefix-hash, r=@alexcrichton

Ignore remap-path-prefix in metadata hash.

Including this flag in the metadata hash causes problems with reproducible builds.

I spent some time considering the different alternatives (such as providing a config option, or an unhashed RUSTFLAGS alternative), and decided this might be the best option.

- It is a very simple, small change.
- It should be safe.
- It is transparent to the user, they don't need to do anything special.
- It doesn't expand Cargo's interface.

Fixes #6914.