Add: always set PERSONAL_DIR "/content_download" in search path (#8481)

This means that if you start OpenTTD with "-c" to indicate another
location to store files, it can still read the content you already
downloaded from your PERSONAL_DIR. This folder is, however,
read-only.

This is useful for situations where you downloaded OpenGFX via
the content-service, but want to run the regression or want to
run with a clean configuration. With this change, you no longer
need to download OpenGFX again.

src/os/windows/win32.cpp

@@ -466,6 +466,10 @@ void DetermineBasePaths(const char *exe)
 		tmp += PERSONAL_DIR;
 		AppendPathSeparator(tmp);
 		_searchpaths[SP_PERSONAL_DIR] = tmp;
+
+		tmp += "content_download";
+		AppendPathSeparator(tmp);
+		_searchpaths[SP_AUTODOWNLOAD_PERSONAL_DIR] = tmp;
 	} else {
 		_searchpaths[SP_PERSONAL_DIR].clear();
 	}