Update INSTALL.md

[csajedi]

apt installing opam on ubuntu is error prone. Use the official installation script instead

[jjcnn]

We should use the official OPAM installation guide (https://opam.ocaml.org/doc/Install.html#Ubuntu), which does indeed list the above solution for Ubuntu version older than 18.04. For 18.04 and newer, though, our installation instructions match the official OPAM recommendation:

add-apt-repository ppa:avsm/ppa
apt update
apt install opam

[jjcnn]

The current instructions match the official installation instructions for OPAM on Ubuntu 18.04 and newer. For older versions, the install script is the recommended solution.

@vaivaswatha : Do you happen to have an old Ubuntu system up and running? If not I can probably set up an old Ubuntu system on a laptop later this week, just to make sure we don't mess something up on older systems.

Alternatively we can skip support for pre-18.04 Ubuntu, but we currently do support it, and this doesn't seem like a strong enough argument to drop the support.

[vaivaswatha]

The instructions we have is valid for 16.04 too: we were using it for Docker builds https://github.com/Zilliqa/scilla/pull/872/files until we moved to 18.04.

I don't see why we shouldn't use the PPA, but I am not opposed to adding a remark explicitly mentioning that for systems older than 18.04, the install script can be used.