1910 use atom image

[PEZ]

What has changed?

The "prescribed" way of running Atom package tests started to fail to often, so now we use the atom-docker-ci Docker image instead and run the tests using that. Much more reliable and since Atom is not developed anymore it should stay stable this way.

Fixes #1910

I also moved the test-grammar job earlier in the ci pipeline so that we don't have to wait for the build step to finish before getting feedback on grammar tests.

My Calva PR Checklist

I have:

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• Made sure I have changed the PR base branch, so that it is not published. (Sorry for the nagging.)
• Updated the [Unreleased] entry in CHANGELOG.md, linking the issue(s) that the PR is addressing.
• Figured if anything about the fix warrants tests on Mac/Linux/Windows/Remote/Whatever, and either tested it there if so, or mentioned it in the PR.
• [ ] Added to or updated docs in this branch, if appropriate
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  • Figured if the change might have some side effects and tested those as well.
  • Smoke tested the extension as such.
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Ping @PEZ, @bpringe, @corasaurus-hex, @Cyrik