gitian: add -D flag to ar for deterministic output for linux deps

ar -D: Operate in deterministic mode. When adding files and the archive
index use zero for UIDs, GIDs, timestamps, and use consistent file modes
for all files.  When this option is used, if ar is used with identical
options and identical input files, multiple runs will create identical
output files regardless of the input files' owners, groups, file modes,
or modification times.