fix sky key secret creation so it won't break release testing locally

[BelSasha]

The problem:
ssh-sky-key is not being deleted during den clean up (which makes sense, it should not have). Therefore, there might be a mismatch between the ssh-sky-key values saved as a den secret, and the local ssh-sky-key values. (For example the ssh-sky-key values saved in den might belong to user A who created and saved this secret for den_tester and the local values might belong to user B who is running release testing with den_tester). This is the explanation to the following issue I was seeing:

ERROR tests/test_resources/test_modules/test_module.py::TestModule::test_module_from_factory[static_cpu_pwd_cluster_den_launcher] - ConnectionError: Could not reach testing-b83cbcb4-14fb-4c7b-ae13-7c73b62b8d2c-20241211-173742-den-aws-cpu-password 44.204.57.6. Is cluster up?

(base) sashabelousovrh@Alexandras-MacBook-Pro runhouse % runhouse server status /den_tester/testing-e91feebf-072b-4350-bb67-f75750c1fdcf-20241211-175620-den-aws-cpu-password

INFO | 2024-12-11 17:58:46 | runhouse.resources.hardware.cluster:1832 | Running command on testing-e91feebf-072b-4350-bb67-f75750c1fdcf-20241211-175620-den-aws-cpu-password: echo "hello"
INFO | 2024-12-11 17:58:47 | runhouse.resources.hardware.cluster:1832 | Running command on testing-e91feebf-072b-4350-bb67-f75750c1fdcf-20241211-175620-den-aws-cpu-password: echo "hello"
Cluster /den_tester/testing-e91feebf-072b-4350-bb67-f75750c1fdcf-20241211-175620-den-aws-cpu-password is not up. If it's an on-demand cluster, you can run `runhouse cluster up 
/den_tester/testing-e91feebf-072b-4350-bb67-f75750c1fdcf-20241211-175620-den-aws-cpu-password` to bring it up automatically.

The fix probably should be creating a new ssh-sky-key each time we are creating a cluster using the test cluster fixtures. This way we could parallelly run tests using den_tester (CI, few runhouse members running release tests locally etc), without getting the above error, since now the values of the local sky key and the one saved in den will match.

[BelSasha]

• Run tests on cpu or gpu clusters #1606
• Update nightly release testing github action #1549
• update cluster tests to use den launcher #1493
• update module tests to use den-launched cluster #1476
• fix sky key secret creation so it won't break release testing locally #1594 👈 (View in Graphite)
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[jlewitt1]

instead of changing logic to accomodate the test, we can save "ssh-sky-key" as a constant, and then just overwrite it in the test

rh.constants.SSH_SKY_KEY = ""