For reproducibility.
Command:
```shell
for file in .github/workflows/*.y*ml; do
npx pin-github-action --comment=' {ref}' "$file"
done
```
Then had to manually replace all the versions with accurate specifiers
(for example, "v4" → "v4.1.1" in case of `actions/checkout`).
We've had a recent PR CI mass failure event, ultimately caused by the
mergeability check GitHub API not returning a result.
But due to the `pkgs/by-name` check workflow not backing off
appropriately between retries, it pummeled the API, resulting in
exceeding the API rate limit:
https://github.com/NixOS/nixpkgs/actions/runs/7010089143/job/19069845070
This commit fixes that for the future by implementing a retry strategy limited to three
retries, with exponential backoff
Previously, even if the check also failed on the base branch, it looked
like the PR introduced the failure.
We can easily have a better error message for such cases.
Meanwhile this also paves the road for something like
https://github.com/NixOS/nixpkgs/issues/256788
This introduces the `pkgs/by-name` directory as proposed by RFC 140.
Included are:
- The implementation to add packages defined in that directory to the
top-level package scope
- Contributer documentation on how to add packages to it
- A GitHub Actions workflow to check the structure of it on all PRs
