dashboard: use macOS 10.15 for race and nocgo darwin builders

A kernel bug in macOS 10.14 is causing a set of tests to be flaky.
Upgrading to macOS 10.15 fixes this issue. This change moves more
testing towards the current version of macOS.

Fixes golang/go#37695

Change-Id: Ide69693e760639a9d680738cad0541d79a0a0443
Reviewed-on: https://go-review.googlesource.com/c/build/+/222238
Run-TryBot: Carlos Amedee <carlos@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

dashboard/builders.go

@@ -2092,13 +2092,13 @@ func init() {
 	})
 	addBuilder(BuildConfig{
 		Name:              "darwin-amd64-nocgo",
-		HostType:          "host-darwin-10_14",
+		HostType:          "host-darwin-10_15",
 		shouldRunDistTest: noTestDir,
 		env:               []string{"CGO_ENABLED=0"},
 	})
 	addBuilder(BuildConfig{
 		Name:              "darwin-amd64-race",
-		HostType:          "host-darwin-10_14",
+		HostType:          "host-darwin-10_15",
 		shouldRunDistTest: macTestPolicy,
 		buildsRepo:        onlyGo,
 	})