Augment dylib search paths for rustc/build scripts

Augment dylib search paths for rustc/build scripts

Whenever a build script or a plugin depends on a native dynamic library, the
dylib search path will be required to include the path to the generated dynamic
library. Currently Cargo does not augment the search path, causing load errors
or execution errors whenever builds are attempted in this case.

This commit remedies the situation by ensuring that dynamic libraries show up in
dylib search paths. This commit is necessary to fully migrate the remaining
tests from the old build command infrastructure to build scripts.