cmd/gopherbot: replace deprecated grpc.WithTimeout
cmd/gopherbot: replace deprecated grpc.WithTimeout

This is deprecated in favor of context.WithTimeout.

Reference:
 https://pkg.go.dev/google.golang.org/grpc@v1.44.0#WithTimeout

Change-Id: Ib4a41a1d56b444f57467076b4e42682ad3478f9f
Reviewed-on: https://go-review.googlesource.com/c/build/+/403684
Run-TryBot: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build