Modify `scripts/check_cla.py` to ignore members of the get-convex GitHub organization

[Nicolapps]

https://github.com/get-convex/convex-helpers/blob/main/.github/workflows/cla.yml calls https://github.com/get-convex/convex-helpers/blob/main/scripts/check_cla.py.

It’s not required to do this check for members of the get-convex GitHub organization. To fix this:

• Modify https://github.com/get-convex/convex-helpers/blob/main/.github/workflows/cla.yml to pass the GitHub username of the PR author as a new environment variable
• Modify https://github.com/get-convex/convex-helpers/blob/main/scripts/check_cla.py to fetch the list of members of the get-convex GitHub organization.
• If the user is a member of the GitHub organization, then skip the usual validation. (Log an appropriate message in the console to explain that validation was skipped because the user works for Convex)
• If the user is NOT a member of get-convex, proceed as usual (i.e. verify that the PR description contains the CLA text)
• If the list of users can't be fetched, log a warning in the console, then proceed as usual