Automatic merge from submit-queue (batch tested with PRs 49222, 49333, 48708, 49337)

Allow to override build date

See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Background: issue #48710
when trying to build the kubernetes package
for the openSUSE Linux distribution
```
/usr/bin/hyperkube
/usr/bin/kubeadm
/usr/src/kubernetes/platforms/linux/amd64/e2e.test
```
had a date+time string embedded in them in the format
2018-08-15T19:08:34Z

which meant that each build produced new binaries
even when nothing actually changed.
Those then had to be synced to mirrors, wasting their and our users' bandwidth.