Code partially adapted from https://github.com/haskell-servant/servant/pull/1627

Code adapted from https://github.com/kazu-yamamoto/logger/pull/205

Code adapted from https://github.com/jacobstanley/unix-compat/pull/60

Code adapted from https://github.com/haskell-streaming/streaming/pull/113

`base-4.18.0.0` introduces `Eq` and `Ord` instances for `Generically1`, which
clash with the ones defined in `generic-data`. Guard the instances in
`generic-data` with CPP to fix this.

On nightly pipelines there are more than 20 jobs so we need to increase
this limit to make sure we get all the jobs we care about.

Fixes GHC #22663

We were doing this very convoluted thing with staged pipelines and
scheduled jobs but it's much more direct to just push to the other
branch directly when there is a job which updates the master branch.

This pipeline is for updating a special branch called "upstream-testing" which is used
when we want to trigger jobs to run on head.hackage. This branch is not protected so
the permissions about who can trigger jobs on upstream-testing are much more liberal than
the master branch.

It turns out that before_script doesn't get combined. Think it's about
time to start generating this CI config as well.. like we do for the
main GHC repo.

It seems that despite the documentation mentioning nothing about it, you
can always save a cache even if a job fails. Which is good for us as we
are caching the nix stuff, which often works when the whole job fails.

See: https://gitlab.com/gitlab-org/gitlab/-/issues/18969

The other option here (which introduces some more complexity) is to
separate the building of the tools into a separate job which then each
runner will download and unpack.

See ghc/ghc#22633

and https://github.com/haskell/directory/issues/147

In cabal 3.8.1.0 --ghc-options specified on command line only passes the options to local packages. Therefore setting `EXTRA_HC_OPTS=-dcore-lint` will only pass the option to the top-level package when it's built but none of the dependencies. This at least wastes work later if we try and recompile a dependency, or worse, it might just be we never compile it with core-lint.

Instead we should modify the `cabal.project` and add a section like:

```
package *
   ghc-options: -dcore-lint
```

Fixes #66

See ghc/ghc#22623 for the assertion
failure

This commit introduces a new variable `SLOW_VALIDATE`, which when set
will try to use a compiler which is built using the `SLOW_VALIDATE`
flavour (which has assertions enabled) rather than a release compiler.

Fixes #62

This should also fix #61 which used to fail because sometimes "latest"
referred to a release pipeline and sometimes a normal validate pipeline.