Use Git JLL for genkeys

[mortenpi]

We're still relying on system Git, which can cause issues (particularly on windows):

DocumenterTools.jl/src/genkeys.jl

Lines 142 to 148 in c1af036

	if Sys.iswindows()
	success(`where where`) || error("'where' not found.")
	success(`where git`) || error("'git' not found.")
	else
	success(`which which`) || error("'which' not found.")
	success(`which git`) || error("'git' not found.")
	end