build: simplify unstable build check

ci.go decides whether a build is unstable by looking at the branch and
tag. This causes issues when a GitHub release is created on the master
branch because the build is considered unstable (the CI environment
reports the branch as "master").

Fix this by looking at the tag only. Any tagged build is stable.

build/ci.go

@@ -459,7 +459,7 @@ func makeWorkdir(wdflag string) string {
 }
 
 func isUnstableBuild(env build.Environment) bool {
-	if env.Branch != "master" && env.Tag != "" {
+	if env.Tag != "" {
 		return false
 	}
 	return true