[gitian-build] Fix repo directory to point at bitcoin-abc

Summary:
The script needs some small fixes to get working out of the box. This is one of them.

Originally reported and fixed by jguthrie7:
https://github.com/Bitcoin-ABC/bitcoin-abc/issues/321
https://github.com/Bitcoin-ABC/bitcoin-abc/pull/320/files

Test Plan:
In a gitian VM as described in /doc/gitian-building:
./bitcoin-abc/contrib/gitian-build.py -c -n -b -o l -j 4 -m 3500 jasonbcox v0.19.10

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3686