[CI] Remove GH variable dependence for tests

[surchs]

Instead of referring to GH variables in the test
workflows, we just hardcode the API URL in the workflow.

This is not ideal, but better than the variable - because variables are not shared with forks.

• Closes CI cannot run on PRs from forks because repo variables are not shared with forks #79

Changes proposed in this pull request:

• replace GH variable with hardcoded API URL in the workflow
• remove the GH variable (manual action, not part of the PR, will happen once merged)

Checklist

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• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see our Contributing Guidelines for more info)
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• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

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[rmanaem]

The changes look good. Feel free to merge after #83 🧑‍🍳
I think the CI prefix would be more appropriate here wdyt?

[surchs]

🚀 PR was released in v0.2.0 🚀