External contributors cannot run CI

[ryscheng]

What is it?

I forgot about our public vars setup when I added a Supabase step a couple weeks ago.
After discussing with @ravenac95 current plan is:

• Remove refresh-test-credentials workflow schedule
• Remove use of public vars in ci-default
• Remove codegen step from the default build. Make it clear in README when you have to codegen
• Change the testing environment to public with no secrets
• Update ci-default to use public and then use pull_request_target

[linear]

OSO-831 External contributors cannot run CI

[ryscheng]

Removing refresh-test-credentials here
#4041

[ryscheng]

Removing codegen from the frontend build here
#4045

[ryscheng]

Updating ci-default here
#4048

[ryscheng]

Fix environment here
#4058

[ryscheng]

Reverting to pull_request here
#4064

[ryscheng]

Re-enabling here
#4068