remapPathPrefixHook: disable automatic path remapping on Darwin builders

[ipetkov]

Motivation

Unfortunately, we cannot support automatic path remapping on Darwin. The remap paths option
requires absolute paths (since rustc will basically do a blind substitution). On Linux builds,
each derivation is chrooted to /build/{name-of-src} which ends up being the same string for
both the real and the deps-only derivations. On Darwin, however, this ends up being
/nix/var/nix/builds/{name-of-derivation} which is different between the main and deps-only
derivations. Since this value ends up in CARGO_BUILD_RUSTFLAGS, it effectively will lead to
cache invalidation when the real derivation runs if the values differ.

Moreover, we can't "just" replace the bytes ourselves since there's no guarantee that the
original build path is the same length as the un-neutered store path. Thus we'll noop this for
now on Darwin builders and leave it up to the caller to handle path remap if needed...

Resolves #944

Checklist

• added tests to verify new behavior
• added an example template or updated an existing one
• updated docs/API.md (or general documentation) with changes
• updated CHANGELOG.md