Automatically select nightly flavour base on the slot name
For example "testing" for slots called "test-*", see LHCbNightlyConf!295 (comment 3107647)
The build flavour should be unset by default, the value should be selected according to the slot name, unless explicitly set, so "test-* goes to "testing", "(lhcb-)release" goes to "release" and everything else to "nightly".