luarocks --verbose install https://jorj.org/bigint-cpp/bigint-1.0.3-1.rockspec
……
os.execute: cd '/tmp/luarocks_bigint-1.0.3-1-4658' && 'git' 'clone' '--branch=v1.0.3' '--depth=1' 'git://github.com/JorjBauer/lua-bigint' 'lua-bigint'
Cloning into 'lua-bigint'...
fatal: Could not read from remote repository.
……

should be https://github.com/JorjBauer/lua-bigint.git