As a last fallback for locating qrenderdoc, return the binary name

* This way if the binary is in the PATH somewhere but not in any of our
  expected locations, we can still execute it.

renderdoc/os/posix/posix_stringio.cpp

@@ -130,8 +130,8 @@ string GetReplayAppFilename()
     }
   }
 
-  // out of ideas
-  return "";
+  // out of ideas, just return the filename and hope it's in PATH
+  return "qrenderdoc";
 }
 
 void GetDefaultFiles(const char *logBaseName, string &capture_filename, string &logging_filename,