dev/sg: 'sg ci' build targeting improvements

Created by: bobheadxi

I want to make sg ci build main-dry-run available for contractors, and also was inspired to give this another pass by @burmudar 's work on sg ci annotations, so did some additional cleanup here alongside the pipeline configuration to improve "build targeting/selecting":

• -pipeline can now be configured (also via env var), for e.g. sourcegraph-contractors
• using -branch, -build for selecting target builds works more consistently now
• fix issue with -wait where it might start selecting the wrong build while polling

Test plan

go run ./dev/sg ci status -p sourcegraph-contractors -branch contractors/SG-36384

and also test out the other sg ci commands