Fix DESI night definition for nightwatch transfers

[weaverba137]

In previous versions of the KPNO nightwatch transfer script, the DESI night was determined via

(dt.datetime.utcnow() - dt.timedelta(7/24+0.5)).strftime('%Y%m%d')

which got changed to

(dt.datetime.now() - dt.timedelta(7/24+0.5)).strftime('%Y%m%d')

while incorporating the nightwatch transfer code into the package. This PR reverts that change.

[coveralls]

Coverage remained the same at 100.0% when pulling 75b766b on nightwatch-night-offset into 9a84866 on master.