dashboard: add nocgo SlowBot alias

Fixes golang/go#42105.

Change-Id: Ia36035e0e86a585d2a1ec5baac451c7219dbda79
Reviewed-on: https://go-review.googlesource.com/c/build/+/264634
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

dashboard/builders.go

@@ -74,6 +74,7 @@ var slowBotAliases = map[string]string{
 	"netbsd-amd64":   "netbsd-amd64-9_0",
 	"netbsd-arm":     "netbsd-arm-bsiegert",
 	"netbsd-arm64":   "netbsd-arm64-bsiegert",
+	"nocgo":          "linux-amd64-nocgo",
 	"openbsd":        "openbsd-amd64-64",
 	"openbsd-386":    "openbsd-386-64",
 	"openbsd-amd64":  "openbsd-amd64-64",