test_LBCORE_716: Use the default linker and not the system linker

Do not add /usr/bin/ and /bin to the cmake.CMakeModules test environment as this potentially pulls in an incompatible linker. In general the environment provided by xenv should be the correct one.

E.g. on my centos7-gcc8 build I got the following error from the test:

/usr/bin/ld: /cvmfs/sft.cern.ch/lcg/releases/gcc/8.2.0-dedd0/x86_64-centos7-gcc62-opt/bin/../lib/gcc/x86_64-pc-linux-gnu/8.2.0/crtbeginS.o: unrecognized relocation (0x2a) in section `.text'
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status
make[3]: *** [lib/libPkgA.so] Error 1
make[2]: *** [PkgA/CMakeFiles/PkgA.dir/all] Error 2
make[1]: *** [all] Error 2