[gitian-build] Bump default memory setting

Summary:
Testing on my VM setup (following the instructions in /doc/gitian-building), I ran into OOM issues.
We also use 3500MB for our TeamCity builds and that appears to work well.

Test Plan:

gitian-build.py -h

# In VM
./bitcoin-abc/contrib/gitian-build.py -c -n -b -o m -j 4 gitianuser v0.19.10

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3688