fix CI dependency on compas

[gonzalocasas]

Fixes https://github.com/compas-dev/compas_fab/issues/388

The CI workflow for IronPython depends on Debian's mirror of PyPi to install wheels, such as compas, because pip generally fails miserably, but now that compas 2.0alpha is released, the debian mirror incorrectly takes that version as /latest and there's no /stable redirect, so, we need to pin to the latest LTS version.

Once compas_fab moves to 2.x, we could unpin this again.

What type of change is this?

• [ ] Bug fix in a backwards-compatible manner.
• [ ] New feature in a backwards-compatible manner.
• [ ] Breaking change: bug fix or new feature that involve incompatible API changes.
• [x] Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [ ] I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• [ ] I ran all tests on my computer and it's all green (i.e. invoke test).
• [ ] I ran lint on my computer and there are no errors (i.e. invoke lint).
• [ ] I added new functions/classes and made them available on a second-level import, e.g. compas_fab.robots.CollisionMesh.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] I have added necessary documentation (if appropriate)

[gonzalocasas]

I guess we cannot do semantic versioning here?

Unfortunately this service (the debian mirror doesn't seem to support it. I tried different variations but nothing worked.