dashboard: enable linux-amd64-stretch builder

The builder named "linux-amd64" was updated from Stretch to Bullseye
in CL 343633, so it's worthwhile for the linux-amd64-stretch builder
to be running post-submit builds.

Updates golang/go#47721.

Change-Id: I5d42af77771789724a5e6ffdc9260ab15e845752
Reviewed-on: https://go-review.googlesource.com/c/build/+/386114
Trust: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: dmitshur

dashboard/builders.go

@@ -1835,10 +1835,9 @@ func init() {
 		Notes: "Runs GOOS=linux but with the Android emulator attached, for running x/mobile host tests.",
 	})
 	addBuilder(BuildConfig{
-		Name:       "linux-amd64-stretch",
-		HostType:   "host-linux-stretch",
-		Notes:      "Debian Stretch. Same as the normal 'linux-amd64' builder at this time, but with -stretch suffix. Used for release builds.",
-		buildsRepo: disabledBuilder, // Disabled because the "linux-amd64" builder does identical work.
+		Name:     "linux-amd64-stretch",
+		HostType: "host-linux-stretch",
+		Notes:    "Debian Stretch.",
 		env: []string{
 			"GO_DISABLE_OUTBOUND_NETWORK=1",
 		},