cmd/internal/osinfo: stop importing golang.org/x/sys/unix

cmd/internal/osinfo: stop importing golang.org/x/sys/unix

This is the only non-vendored file that imports x/sys/unix.
Switch to fetching the information in this package.

Change-Id: I4e54c2cd8b4953066e2bee42922f35c387fb43e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/623435
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>