dashboard: require at least Go 1.13 for darwin-386 builder

release-branch.go1.12 has failures for darwin-386-10_14, which are fixed
in Go 1.13. darwin-386 is not a first-class port and on a deprecated
architecture for macOS, and can be skipped on older branches.

Fixes golang/go#33756

Change-Id: I5f2e95fe9515978a85b986787d8126655f849629
Reviewed-on: https://go-review.googlesource.com/c/build/+/191878
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

dashboard/builders.go

@@ -1914,7 +1914,9 @@ func init() {
 		Name:              "darwin-386-10_14",
 		HostType:          "host-darwin-10_14",
 		shouldRunDistTest: macTestPolicy,
-		buildsRepo:        onlyGo,
+		buildsRepo: func(repo, branch, goBranch string) bool {
+			return repo == "go" && atLeastGo1(branch, 13)
+		},
 		MaxAtOnce: 1,
 		env:       []string{"GOARCH=386", "GOHOSTARCH=386"},
 	})

dashboard/builders_test.go

@@ -467,8 +467,8 @@ func TestBuilderConfig(t *testing.T) {
 		{b("darwin-amd64-10_10@go1.12", "go"), onlyPost},
 		{b("darwin-amd64-10_10@go1.11", "go"), onlyPost},
 		{b("darwin-386-10_14", "go"), onlyPost},
-		{b("darwin-386-10_14@go1.12", "go"), onlyPost},
-		{b("darwin-386-10_14@go1.11", "go"), onlyPost},
+		{b("darwin-386-10_14@go1.12", "go"), none},
+		{b("darwin-386-10_14@go1.13", "go"), onlyPost},
 
 		// plan9 only lived at master. We didn't support any past releases.
 		// But it's off for now as it's always failing.