Add action to auto-update mamba

[xhochy]

Fixes #116

This checks every 6h whether there is a new conda / mamba release and makes a PR that updates the Miniforge3/construct.yaml file.

Important things:

• Only opens a single PR and only if there are changes. If there is an open PR with conflicting changes, it won't do anything.
• If the PR was closed, it will delete the branch and recreate it with new commits.
• The commit will be pushed using a repo-specific deploy key. If the commit were pushed using the normal ${{ GITHUB_TOKEN }}, CI wouldn't be triggered. As the deploy key is specific to this repo, it doesn't give anyone in the miniforge team more permissions that they would already have.
• I have added the deploy key to the repo and will also add it in the usual place in keybase.

See xhochy/miniforge-bot#3 for a working example of this change.

[isuruf]

For a new conda version, this would require 3 * 20 CI jobs instead of 20 CI jobs?

[xhochy]

For a new conda version, this would require 3 * 20 CI jobs instead of 20 CI jobs?

Yes, this is the cost of this automation but it also means that we tests were run before we set the tag.

[xhochy]

@isuruf Anything I can do to get this merged?

[isuruf]

Can we do it for mamba version only? (Maybe open an issue for conda version. I don't see any point to creating a PR, merging it and tagging vs just tagging)

[xhochy]

Maybe open an issue for conda version

You mean that we could have a bot that opens an issue for a new conda version?

[isuruf]

You mean that we could have a bot that opens an issue for a new conda version?

Yes an issue asking for a new tag.

[xhochy]

Added an action for a tag over at #132

Can this here be merged, too?