chore: remove pyramation/node-sqitch container, use ubuntu-latest directly

[pyramation]

Summary

Removes the pyramation/node-sqitch:20.12.0 container from CI workflows, allowing jobs to run directly on ubuntu-latest. This simplifies the CI setup as the custom container is no longer needed.

Changes:

• Removed container: pyramation/node-sqitch:20.12.0 directive from both workflows
• Fixed PGHOST from pg_db to localhost (required since jobs now run directly on the runner, not inside a container)
• Added PGPM_VERSION env variable (2.7.9) as single source of truth for pgpm version
• Added pgpm CLI caching using actions/cache@v4
• Pinned pgpm install to use the version from env variable

Files changed:

• .github/workflows/ci.yml
• .github/workflows/integration-test.yml

Review & Testing Checklist for Human

• Verify CI passes - Confirm all test jobs complete successfully with the new configuration
• Check PostgreSQL connectivity - The PGHOST: localhost change is critical for connecting to the pg_db service container from the runner. Verify database operations work correctly.
• Verify pgpm version 2.7.9 compatibility - Ensure this version works with the existing test suite and test-packages --full-cycle command

Notes

This is part of a broader simplification effort across multiple repos (pgpm-modules, supabase-test-suite, pgpm-boilerplates).

Link to Devin run: https://app.devin.ai/sessions/d84286973f0a46119274e37169001af9
Requested by: Dan Lynch (@pyramation)

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