cmd: be more lenient when --lua-version can't find an interpreter
cmd: be more lenient when --lua-version can't find an interpreter

This will still allow things like `luarocks list` and
`luarocks search` to work.

luarocks/luarocks,tarantool/luarocks,keplerproject/luarocks,keplerproject/luarocks,keplerproject/luarocks,tarantool/luarocks,luarocks/luarocks,keplerproject/luarocks,luarocks/luarocks,tarantool/luarocks