gitlab-ci: Rework job predicates

See merge request !127

Previously we were running the `master`, `build-8.10`, `build-9.0` jobs even
on pipelines triggered for a particular commit from the GHC repository.

Adapt singletons patches to Nat/Natural being merged on HEAD

See merge request !126

Add text-format patch

See merge request !125

Remove some unneeded patches, migrate some patches to their latest Hackage versions

See merge request !124

This allows it to build with `th-abstraction-0.4.*`, which is required for
build plans that `head.hackage`'s CI needs to use.

Their latest Hackage versions all support GHC 9.0 and HEAD.

ci: Add hgmp to the default build

See merge request !102

Drop singletons-2.6

Closes ghc#18748

See merge request !123

Fixes ghc#18748.

Add "quick" CI mode

See merge request !97

Build with -ddump-timings

See merge request !122

Update README.md: Add repotitory stanza to readme. I always look it up here, and fail to find.

See merge request !96

The only consumer of the GHC_TARBALL envvar is ghc-artefact-nix.

To be used by validation builds for compiler performance characterisation.

ci: Preserve compiler info

See merge request !94

I always look it up here, and fail to find.

These all now build without issues on GHC HEAD.

The types of RawIO functions changed: A Word64 offset was added.

Additionally there was a revision of the package dependencies on
hackage.