internal/testenv: treat linux-riscv64-unmatched as a slow builder
internal/testenv: treat linux-riscv64-unmatched as a slow builder

For golang/go#49321

Change-Id: I36e1b26a52956b6b2a9a2a655934aba853759162
Reviewed-on: https://go-review.googlesource.com/c/tools/+/361155
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools