Implement a workaround for DYLD_LIBRARY_PATH in lre-rs

[aaronmondal]

See: https://github.com/bazelbuild/rules_rust/issues/2331

Rustc will wrongly try to look for its driver lib in a cas-relative path. That's wrong. As a workaround we can currently use --action_env=DYLIB_LIBRARY_PATH for MacOS with RBE but we can integrate this into the toolchain to make it automatic.