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/139426Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/x509/root_darwin.go

@@ -16,7 +16,6 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
-	"os/user"
 	"path/filepath"
 	"strings"
 	"sync"
@@ -67,17 +66,17 @@ func execSecurityRoots() (*CertPool, error) {
 		"/Library/Keychains/System.keychain",
 	}
 
-	u, err := user.Current()
-	if err != nil {
+	home := os.UserHomeDir()
+	if home == "" {
 		if debugExecDarwinRoots {
-			println(fmt.Sprintf("crypto/x509: get current user: %v", err))
+			println("crypto/x509: can't get user home directory")
 		}
 	} else {
 		args = append(args,
-			filepath.Join(u.HomeDir, "/Library/Keychains/login.keychain"),
+			filepath.Join(home, "/Library/Keychains/login.keychain"),
 
 			// Fresh installs of Sierra use a slightly different path for the login keychain
-			filepath.Join(u.HomeDir, "/Library/Keychains/login.keychain-db"),
+			filepath.Join(home, "/Library/Keychains/login.keychain-db"),
 		)
 	}