Add shared gke server with postgres backend test to nightly build

[zpoint]

Need to work with this PR: https://github.com/assemble-org/prototype/pull/94

test pass: https://buildkite.com/skypilot-1/nightly-build-shared-gke-api-server/builds/4#_

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)