nix hash convert: Support SRI hashes that lack trailing '=' characters (backport #12013)

[mergify]

Motivation

Fixes #11996, #11997.

Also, don't fail on uppercase base-16 hashes.

Context

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

---

This is an automatic backport of pull request #12013 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 408c2fa has failed:

On branch mergify/bp/2.25-maintenance/pr-12013
Your branch is ahead of 'origin/2.25-maintenance' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 408c2faf9.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/nix/hash.cc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[mergify]

This pull request has been removed from the queue for the following reason: pull request dequeued.

Pull request #12018 has been dequeued. Rebase merges are not allowed on this repository.. The repository configuration doesn't allow rebase merge. The merge method configured in Mergify configuration must be allowed in the repository configuration settings..

You should look at the reason for the failure and decide if the pull request needs to be fixed or if you want to requeue it.

If you want to requeue this pull request, you need to post a comment with the text: @mergifyio requeue