ci: pin poetry version to 1.8.5

[Alex-Bair]

Description:

CI for source-criteo and source-shopify started failing sometime between 04JAN2025 and 06JAN2025. The logs for one of the most recent successful runs on 04JAN contain:

Warning: The current project could not be installed: No file/folder found for package source-shopify-estuary
If you do not want to install the current project use --no-root.
If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.
In a future version of Poetry this warning will become an error!

That successful run was using Poetry v1.8.5.

On 05JAN2025, Poetry released v2.0.0, and the snok/install-poetry@v1 action began installing v2.0.0 instead of v1.8.5. True to it's earlier warning, Poetry started raising an error during CI for source-criteo and source-shopify & CI for those connectors started failing.

It looks like pinning the Poetry version used during install to v1.8.5 lets CI for those two connectors succeed again.

Workflow steps:

(How does one use this feature, and how has it changed)

Documentation links affected:

(list any documentation links that you created, or existing ones that you've identified as needing updates, along with a brief description)

Notes for reviewers:

(anything that might help someone review this PR)

---

This change is 

[Alex-Bair]

Closing since Will's approach to fix the connector build in #2444 to work with Poetry v2.0.0 is better.