Change Acts lib install dir, enable MaterialPlugin

This MR adds the cmake flags

-DACTS_BUILD_MATERIAL_PLUGIN:STRING=ON
-DCMAKE_INSTALL_LIBDIR:PATH=lib

to the Acts ExternalProject build, which makes the build output shared libs into lib/ instead of lib64/ and enables the build of the material plugin.

Also, in line with the changes introduced by acts-core!434, the naming of the package is changed to Acts.