Bug#1066083: gnome-maps: please make the build reproducible

[James Addison]

Source: gnome-maps
Followup-For: Bug #1066083
X-Debbugs-Cc: lamby at debian.org
Control: forwarded -1 https://gitlab.gnome.org/GNOME/gnome-maps/-/merge_requests/453

Although a SOURCE_DATE_EPOCH-based approach was suggested in this bugreport,
I've independently opened an alternative that removes the dynamic build-date
timestamp entirely upstream, with reference to the previously-mentioned
merge request in Gnome's GTK that follows the same approach.

I'm OK with either approach, but felt that opening an upstream merge request
could be worthwhile here.