tests: Abstract the docker image tag for Nightly tests

This abstraction allows us to more easily change the tags between
different gem5 releases and/or between stable and the develop branch.

Change-Id: Ieb38e9a333ef8592b586014a3a9220f4a18c64e3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65919
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed