Handle external tools paths more gracefully

[azubieta]

https://github.com/linuxdeploy/linuxdeploy-plugin-qt/blob/5d2191e43e40409244456605cfd42e5f029e5d38/src/main.cpp#L508-L512

Relying on the environment variable PATH to resolve the external tools paths is unsafe instead we should resolve them manually based on the qmake configuration.