dashboard: add linux-amd64-staticlockranking builder

This is a new linux/amd64 GCE builder for the purpose of
testing the GOEXPERIMENT=staticlockranking configuration
which is landing to master branch soon.

It runs only on the main Go repository master branch and
release branches 1.15 and newer. No golang.org/x repos.
Not included in trybots by default, but can be requested
via SlowBots by saying TRY=linux-amd64-staticlockranking.

Also expand documentation of BuildConfig.buildsRepo field
to make it more clear how to use it. A common mistake for
custom buildsRepo functions that are added is to create a
custom policy with a minor adjustment and forgetting to
preserve the rest of the default build repo policy.

Fixes golang/go#37937

Change-Id: Ic56f82e9ede30cb6fe8f6cc7d0282d6a0a12c1b8
Reviewed-on: https://go-review.googlesource.com/c/build/+/224078
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/dashboard/builders_test.go b/dashboard/builders_test.go
index 5685708..38b4d5d 100644
--- a/dashboard/builders_test.go
+++ b/dashboard/builders_test.go
@@ -557,6 +557,11 @@
 		{b("dragonfly-amd64-5_6", "go"), onlyPost},
 		{b("dragonfly-amd64-5_6", "net"), onlyPost},
 		{b("dragonfly-amd64-5_6@go1.13", "net"), onlyPost},
+
+		{b("linux-amd64-staticlockranking", "go"), onlyPost},
+		{b("linux-amd64-staticlockranking@go1.15", "go"), onlyPost},
+		{b("linux-amd64-staticlockranking@go1.14", "go"), none},
+		{b("linux-amd64-staticlockranking", "net"), none},
 	}
 	for _, tt := range tests {
 		t.Run(tt.br.testName, func(t *testing.T) {