config/credentials: don't run 'pass' to detect it

'CheckInitialized' in the credential-helper library actually invokes
`pass`, which isn't desirable (see #699).

This moves the check to be simpler, and then pass will only be invoked
when it's needed (such as for `docker login` or when pulling from a
private registry).

This logic could also reasonably live in the credential-helper library,
but it's simple enough it seems fine in either location.

Signed-off-by: Euan Kemp <euank@euank.com>

cli/config/credentials/default_store_linux.go

@@ -1,12 +1,11 @@
 package credentials
 
 import (
-	"github.com/docker/docker-credential-helpers/pass"
+	"os/exec"
 )
 
 func defaultCredentialsStore() string {
-	passStore := pass.Pass{}
-	if passStore.CheckInitialized() {
+	if _, err := exec.LookPath("pass"); err == nil {
 		return "pass"
 	}