Don't build master of goxcrypto, goxnet, uniuri and go-webrtc

During the review of #29143 (moved) we discussed in which cases a component in nightly builds should use the master commit. The conclusion was that we usually want to use master for the things we develop, but not for external components.

goxcrypto, goxnet, uniuri are external components that are currently built using master. go-webrtc is not but has a master target that looks like if it was named master instead of nightly as an error.