google: prefer os.Getenv("HOME") over os/user.Current() so as to avoid SEGV

Due to an issue in handling thread-local storages, os/user can lead to SEGV
when glibc is statically linked with.

So we prefer os.Getenv("HOME") for guessing where is the home directory.

See also: golang/go#13470

Change-Id: I1046ff93a71aa3b11299f7e6cf65ff7b1fb07eb9
Reviewed-on: https://go-review.googlesource.com/34175
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

google/sdk.go

@@ -160,9 +160,13 @@ var sdkConfigPath = func() (string, error) {
 }
 
 func guessUnixHomeDir() string {
-	usr, err := user.Current()
+	// Prefer $HOME over user.Current due to glibc bug: golang.org/issue/13470
-	if err == nil {
+	if v := os.Getenv("HOME"); v != "" {
-		return usr.HomeDir
+		return v
 	}
-	return os.Getenv("HOME")
+	// Else, fall back to user.Current:
+	if u, err := user.Current(); err == nil {
+		return u.HomeDir
+	}
+	return ""
 }