Herwig7_i CMake Fix, master branch (2021.07.22.)

I came across this problem while doing something entirely different. I had to realise that in our current nightlies the powhegHerwig library is only working by chance...

You see, the ThePEG generator depends publicly on GSL.

https://thepeg.hepforge.org/doxygen/RandomGenerator_8h_source.html

The reason that our current nightlies are not freaking out about this, is that lxplus nodes come with GSL installed on the operating system under /usr. But this now means that our build picks up GSL from /usr/include/gsl/, and then links our code against a ThePEG library that itself was linked against LCG's GSL library earlier. I guess, by chance, this works for the moment. But this was clearly a bug. We should use LCG's GSL installation here.

The way I found this was by building Athena inside of a Docker image that did not have GSL installed in it...