tests: Unset SOURCE_DATE_EPOCH

tests: Unset SOURCE_DATE_EPOCH

Some distributions set this during build in order to have reproducible
builds from the same source code: for example, Debian uses the date
from debian/changelog.

However, some of our tests assume that `ostree commit` will result in
a commit with the current date/time, and SOURCE_DATE_EPOCH breaks that
assumption. Unset it for our build-time tests.

Resolves: https://github.com/ostreedev/ostree/issues/2405
Signed-off-by: Simon McVittie <smcv@collabora.com>