We partially revert 8dbd2edd98 by removing
all instances of setting `GIT_DIR` in the gitian descriptors. Currently,
gitian builds cause zcash to have version strings which look like
`Zcash Daemon version v5.3.0-35186b009-dirty`, which is inaccurate.
Setting `GIT_DIR` with a current working directory of
`/home/debian/build/zcash/distsrc-x86_64-linux-gnu` makes the git
invocations in `genbuild.sh` see that directory as the top of the
working tree, which causes git to believe that a bunch of tracked files
were deleted (namely, those not included by the `make dist` step).
This causes `genbuild.sh` to believe that it is working with a dirty
tree, and with the help of `src/clientversion.cpp`, zcash gets a version
string that includes a hash ending in `-dirty`.
The release commit was built with a `make-release.py` that was before the parallel-gitian commit, causing the parallel gitian descriptor to not be updated.