dashboard: enable snapshot for linux-riscv64-unmatched

linux-riscv64-unmatched almost stalled by sub-repo builds (> 1000).
We might speed it up by enable snapshot although it has a slow uplink.

Change-Id: I904a235498e4c35b1e1ebe8bea960fe2fb03dded
Reviewed-on: https://go-review.googlesource.com/c/build/+/349029
Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2452,7 +2452,6 @@ func init() {
 		Name:           "linux-riscv64-unmatched",
 		env:            []string{"GO_TEST_TIMEOUT_SCALE=4"},
 		FlakyNet:       true,
-		SkipSnapshot:   true, // The builder has a slow uplink bandwidth.
 		buildsRepo:     onlyMasterDefault,
 		distTestAdjust: riscvDistTestPolicy,
 	})