LD_LIBRARY_PATH doesn't get passed to package

[Chris Austen]

There are times where the LD path is needed by the repo when building (i.e. ASAN) but it seems cget wipes the path out
https://github.com/pfultz2/cget/blob/master/cget/prefix.py#L111

[Krzysztof Drewniak]

The expected behavior would be to prepend to LD_LIBRARY_PATH if it's set, not overwrite it