os/user: fix buffer retry loop on macOS

os/user: fix buffer retry loop on macOS

getpwnam_r and friends return the errno as the result,
not in the global errno. The code changes in CL 449316
inadvertently started using the global errno.
So if a lookup didn't fit in the first buffer size,
it was treated as not found instead of growing the buffer.

Fixes #56942.

Change-Id: Ic5904fbeb31161bccd858e5adb987e919fb3e9d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/455815
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>