drive: fix crash when trying to creating shortcut to a Google doc

Before this change, we uses an unguarded type assertion which failed
when NewObject returned a google document instead of an Object.

This change uses the ID() method to read the id which works for all
types of file instead and returns a sensible error if one isn't found.

Fixes #8990

backend/drive/drive.go

@@ -3396,9 +3396,11 @@ func (f *Fs) makeShortcut(ctx context.Context, srcPath string, dstFs *Fs, dstPat
 			return nil, fmt.Errorf("failed to find source dir: %w", err)
 		}
 		isDir = true
-	} else {
+	} else if do, ok := srcObj.(fs.IDer); ok {
 		// source was a file
-		srcID = srcObj.(*Object).id
+		srcID = do.ID()
+	} else {
+		return nil, fmt.Errorf("unknown source object: %T", srcObj)
 	}
 	srcID = actualID(srcID) // link to underlying object not to shortcut