[Core] Set file permission when writing the ssh config file

[Michaelvll]

We now explicitly set the permission for the ssh config file writen by SkyPilot, because sometimes if the user changed the username of their local machine, and still use the conda environment with skypilot installed under the old /home/old-user folder. In this case, the files writen to ~/.sky/generated/ssh can have a permssion dwxr-rw-rw-- which violates the permission required for ssh, causing all ssh fails.

This is one of the potential reasons for #2689. This PR also prints out the output of the last ssh command if the wait ssh timeout.

Reproducible steps:

1. docker run -it --rm centos:centos7 /bin/bash
2. adduser -m test-original; adduser -m test-new
3. passwd test-original; passwd test-new
4. sermod -aG wheel test-new; sermod -aG wheel test-original
5. su test-original; # setup miniconda
6. setfacl -R -m u:test-new:rwx /home/test-original/
7. su test-new; /home/test-original/miniconda3/bin/conda init; # install skypilot
8. sky launch --cloud gcp --cpus 2 echo hi; # task submission fails due to ssh permission issue with the folder

Bad owner or permissions on /home/test-new/.sky/generated/ssh/test-new-user

By deleting ~/.sky/generated/ssh and switch to this PR, the problem goes away.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh