crypto/x509: fix getting user home dir on darwin

As pointed out in https://github.com/golang/go/issues/26463,
HOME (or equivalent) environment variable (rather than the
value obtained by parsing /etc/passwd or the like) should be
used to obtain user's home directory.

Since commit fa1a49aa there's a method to obtain
user's home directory -- use it here.

Change-Id: I852fbb24249bcfe08f3874fae6e7b9d01d869190
Reviewed-on: https://go-review.googlesource.com/c/139426


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>