cmd/rmplaysnippet: account for local datastore host env var
cmd/rmplaysnippet: account for local datastore host env var

If DATASTORE_EMULATOR_HOST is set, datastore.NewClient will use
it to connect to a locally-running datastore instance. Clear it
before creating the client and set a 30-second timeout on the
context so that the process doesn't appear to hang indefinitely
when the env var is set.

Change-Id: I605ee699a55f4537c174a5667c92044936f10d74
Reviewed-on: https://go-review.googlesource.com/c/build/+/185142
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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