dashboard: add a known issue for linux-riscv64-unmatched

For golang/go#53379.

Change-Id: I758dfac4bf794de151d9c5105954e68224d955b7
Reviewed-on: https://go-review.googlesource.com/c/build/+/412175
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 539693a..dd92174 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -2646,6 +2646,7 @@
 		buildsRepo:     onlyMasterDefault,
 		distTestAdjust: riscvDistTestPolicy,
 		privateGoProxy: true, // this builder is behind firewall
+		KnownIssues:    []int{53379},
 	})
 	addBuilder(BuildConfig{
 		Name:           "linux-s390x-ibm",