Use silkworm submodule library when building evm_runtime contract unit_tests

[elmato]

First, leap should be able to be built using c++20.
AntelopeIO/leap#1079

[stephenpdeos]

We would like to use the copy of silkworm we use in building our contracts for building our tests as well. (Right now we use a separate copy because of a limitation of not being able to use C++20 in libtester, but that limitation is removed with the libtester pulled in from the Leap 5 dependency.)