drive: fix listing of the root directory with drive.files scope

We attempt to find the ID of the root folder by doing a GET on the
folder ID "root". With scope "drive.files" this fails with a 404
message.

After this change if we get the 404 message, we just carry on using
"root" as the root folder ID and we cache that for future lookups.

This means that changenotify messages will not work correctly in the
root folder but otherwise has minor consequences.

See: https://forum.rclone.org/t/fresh-raspberry-pi-build-google-drive-404-error-failed-to-ls-googleapi-error-404-file-not-found/12791

backend/drive/drive.go

@@ -1030,7 +1030,13 @@ func NewFs(name, path string, m configmap.Mapper) (fs.Fs, error) {
 		// Look up the root ID and cache it in the config
 		rootID, err := f.getRootID()
 		if err != nil {
-			return nil, err
+			if gerr, ok := errors.Cause(err).(*googleapi.Error); ok && gerr.Code == 404 {
+				// 404 means that this scope does not have permission to get the
+				// root so just use "root"
+				rootID = "root"
+			} else {
+				return nil, err
+			}
 		}
 		f.rootFolderID = rootID
 		m.Set("root_folder_id", rootID)