dashboard: restrict linux-ppc64le-power10-osu to go1.20 and newer

GOPPC64=power10 is only supported by go1.20 and newer. Don't use
this builder with older releases.

Change-Id: I5dc012165fce32f13aa8316b54f0fbe9ef337b61
Reviewed-on: https://go-review.googlesource.com/c/build/+/456695
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index cd03011..137670a 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -2657,6 +2657,9 @@
 		FlakyNet:       true,
 		distTestAdjust: ppc64DistTestPolicy,
 		env:            []string{"GO_TEST_TIMEOUT_SCALE=2"}, // see go.dev/issues/44422
+		buildsRepo: func(repo, branch, goBranch string) bool {
+			return atLeastGo1(goBranch, 20) && buildRepoByDefault(repo)
+		},
 	})
 	addBuilder(BuildConfig{
 		Name:     "linux-arm64-aws",