[WIP] Update test using mysql 5.7 driver to use ubuntu 20.04

[frank-emrich]

See my first comment below.

[jamescheney]

So is OCaml 5.7 not supported as of Ubuntu >=20?

Using mariadb should be fine, in fact @SimonJF had had in mind to try to use it for comparing temporal features, but I made an attempt or two to try it using hte MySQL driver and didn't get anywhere (possibly because I didn't know what I was doing).

[frank-emrich]

Sorry, I created this as a playground to see how the CI would react to certain changes, but hadn't added a description to the PR yet.

The problem isn't using MySQL 5.7 per se, but only the client packages used on the OCaml side.

In Links, we currently have two "drivers" for talking to mysql servers. These are basically thin wrappers around client libraries for talking to mysql servers.

1. The first one is provided by our own links-mysql package and in turn relies on the mysql opam package.
2. The second one is provided by our links-mysql8 package and in turn relies on the mysql8 opam package.

Here, the mysql8 opam package is a fork of the mysql opam package to support the mysql client version 8.0. Note that the mysql 8 clients can still talk to servers using older versions, in particular 5.7. Our test suite currently covers this in the tests called "Links main workflow / links-mysql8_mysql57". These use links-mysql8, which in turn uses the mysql8 opam package to talk to a mysql 5.7 server.

What broke is the links-mysql/mysql package (without 8 in the name): It fails to build against client version 8, which is the one provided on newer Ubuntu versions. We kept this driver around in addition to links-mysql8/mysql8 hoping that the original mysql package would eventually gain support for version 8 so that there would only be a single, canonical mysql package again in the Ocaml ecosystem. However it seems like this hasn't happened so far.

What I've tried in this PR are some workarounds to make the old mysql opam package compile against and use newer client versions, but this seems to have failed.

We should probably abandon our own links-mysql package for the time being (until it may get patched some day) and only supportlinks-mysql8 going forward. Given that this client can still talk to 5.7 servers, we do not really lose any functionality this way.

[frank-emrich]

Subsumed by #1178