dashboard: skip perf test for linux-riscv64-unmatched

The perf test still timeed out even CL 417314 increased builder
timeout scale up to 6.
Unfortunately we had to skip this test for now and revert timeout
scale back to 4.

For golang/go#53745

Change-Id: I247179b42c575ca05bb0ec6b51d260ab51d912f3
Reviewed-on: https://go-review.googlesource.com/c/build/+/417495
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2713,11 +2713,17 @@ func init() {
 	addBuilder(BuildConfig{
 		HostType:       "host-linux-riscv64-unmatched",
 		Name:           "linux-riscv64-unmatched",
-		env:            []string{"GO_TEST_TIMEOUT_SCALE=6"},
+		env:            []string{"GO_TEST_TIMEOUT_SCALE=4"},
 		FlakyNet:       true,
-		buildsRepo:     onlyMasterDefault,
 		distTestAdjust: riscvDistTestPolicy,
 		privateGoProxy: true, // this builder is behind firewall
+		buildsRepo: func(repo, branch, goBranch string) bool {
+			// see https://go.dev/issue/53745
+			if repo == "perf" {
+				return false
+			}
+			return onlyMasterDefault(repo, branch, goBranch)
+		},
 	})
 	addBuilder(BuildConfig{
 		Name:           "linux-s390x-ibm",