sweet/common: invoke go env with -json flag

The go command supports the -json flag to make it convenient to parse
its output programmatically. Since human-readable output isn't needed
here, switch to using the -json flag.

Change-Id: I2e25a96a020cff5d8a99c152bd929f06f6920c42
Reviewed-on: https://go-review.googlesource.com/c/benchmarks/+/758680
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>