Pin the GitHub Linux machines to 20.04

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Some of the workflows were using ubuntu-latest, which has changed from 20 to 22. Pin all the workflows to 20, until we want to update all of them to 22 (and solve the issues that come with that).

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/12658363813

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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