build-and-test-all: Build DNSdist on Ubicloud runners when needed

build-and-test-all: Build DNSdist on Ubicloud runners when needed

GH actions provides us with a 16 GB runner on public repositories,
but only 7 GB on private ones. Unfortunately our current workflow
assumes that we can get away with 4 concurrent jobs when building
DNSdist, which is true when we have 16 GB available but not with
7 GB.
So this commit switches to Ubicloud runners (standard 4, 4 vCPU,
16 GB) for repositories defining the `REPOSITORY_USE_UBICLOUD`
variable to 1. These runners are also significantly faster than the
GH actions ones.