- adjust reviewers
- add workflow as cache key
- install attr only when tesitng
- fix s390x workflow by checking out the merge PR instead of master

This will run the three test suites (unit, functional and old) in
parallel, meaning that neovim is built for each test and run separately.
This has a slight increase in total CI usage, but it allows rerunning
only the specific test suite that failed for flaky tests, which will
save some time.

Ideally we'd remove any drawbacks by building neovim once and reusing it
for each test suite, but that is not currently possible due to poor
upload/download speeds of the upload-artifact and download-artifact
actions. This has been addressed in
https://github.com/actions/toolkit/pull/1488, but will only be made
available in upload-artifact@v4 and download-artifact@v4.

- add reviewers
- correct cache key

This will ensure the cache isn't used when an image upgrade changes the
compiler version, causing the build to fail.

The ci/ directory is now only used for Cirrus, not for GitHub Actions.

If one uses .deps when DEPS_BUILD_DIR is defined in another location it
leads to very surprising behaviors, as it looks for libraries in other
places other than .deps.

Having a workflow that only builds neovim without running all of the
tests is a cheap way to test the build still works without burning too
much CI time.

Using the base branch as cache means that pull requests won't be able to
use the cache from the master branch, since the master branch cache
doesn't have a base_ref as it's generated from a push. Removing base_ref
makes the cache key from master and PR branch the same, provided the any
build files don't change.

The advantages of using an action is that it reduces boilerplate and
repetition from the main ci.yml workflow.