cmd/go/internal/bug: use envcmd instead of go env

cmd/go/internal/bug: use envcmd instead of go env

Add the printGoEnv function to print the go environment variables, using
the envcmd package instead of invoking go env.

Add the PrintEnv function to the envcmd package, to avoid duplicating
code.

Updates #45803

Change-Id: I38d5b936c0ebb16e741ffbee4309b95d6d0ecc6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/314230
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>