rustpkg test package_script_with_default_build makes bogus assumptions about the build directory layout

[msullivan]

The test package_script_with_default_build in rustpkg is busted. It needs to access a file in the source distribution, which it finds based on the test executable path. This doesn't work because it relies on a particular directory layout of the build directory. (Namely, that the root build directory is a subdirectory inside the source directory.)

[huonw]

triage, test is still #[ignore]'d.

[thestinger]

rustpkg is gone