make: allow setting the timestamp used to generate artifact names

[dch]

Summary

• in pkgbase
• in .snapYYYYMMDDHHSS

Release builds are run in parallel, but the same source build on different
architectures will produce different names, despite arguably being built
at more or less the same time.

make REPRODUCIBLE_EPOCH=1746648220 ... cool stuff

[dch]

as mentioned on my email to re@ this would be handy for the multi-arch
container builds as well as possibly for general reproducible builds.

Would like some brief feedback before I try this, it takes over a
day to run thermite locally to find out the result.

[dch]

handle case when var is *not* set and bmake complains about it