cmd/buildlet/stage0: define env for linux-riscv64-unmatched

This CL sets up the environment for `host-linux-riscv64-unmatched`

For golang/go#53379

Change-Id: I6b24dabb9407cb98e095c555001adc5a022d1d66
Reviewed-on: https://go-review.googlesource.com/c/build/+/413334
Auto-Submit: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

cmd/buildlet/stage0/stage0.go

@@ -173,7 +173,7 @@ Download:
 		cmd.Args = append(cmd.Args, reverseHostTypeArgs(buildEnv)...)
 	case "host-linux-ppc64-osu":
 		cmd.Args = append(cmd.Args, reverseHostTypeArgs(buildEnv)...)
-	case "host-linux-amd64-wsl":
+	case "host-linux-amd64-wsl", "host-linux-riscv64-unmatched":
 		cmd.Args = append(cmd.Args, reverseHostTypeArgs(buildEnv)...)
 	}
 	switch osArch {