…arch paths.

This drops `native_dirs` entries that are not within the target directory when
modifying the (DY)LD_LIBRARY_PATH environment variable before running
programs.

For the library paths to be passed through, they must reside within the build
output directory.

I was just pasting the build directories into Rust string literals, so Windows
paths with backslashes were being interpreted as having unknown string
escapes. Raw string guards should fix this for all but the most pathological
of build directories.