cmd/bent: inherit http_proxy variable

Change-Id: I6d22487c8967cdc92b84a31f622c9d6c3eface1f
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/565635
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>