[rootx] Do not overwrite existing LD_LIBRARY_PATH!
[rootx] Do not overwrite existing LD_LIBRARY_PATH!

See https://root-forum.cern.ch/t/ld-library-path-is-ignored/38378
Fixes https://github.com/root-project/root/pull/4703/files#r366820344

root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,karies/root