[Lit] Pass through DFLTCC from the surrounding environment

Many distros ship zlib with the IBM Z deflate hardware acceleration
patch [1]. Sometimes it's desirable to disable the acceleration, for
example, for reproducibility. This can be done by exporting DFLTCC=0.
llvm-lit clears this environment variable, which causes
compress-debug-sections-zlib.test fail on z15 and later machines. Add
DFLTCC to the list of variables to keep.

[1] https://github.com/madler/zlib/pull/410

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D130253