Search for and link to libsodium in standalone build

The LCG libzeromq is linked to libsodium, but pkg-config doesn't seem to propagate this, so explicitly search for and link to it. libsodium seems to be installed as a system library in all test systems, so this was not observed before.

LHCb!3423 (merged) takes care of the stack build, this MR is for the standalone build.

added Build only GitLab CI labels

added RTA label

added cleanup label

merged

mentioned in commit 78aa1b70

mentioned in issue Moore#390 (closed)