Move model filelocks from /tmp/ to ~/.cache/vllm/locks/ dir

[mgoin]

Creating and accessing filelocks in the global /tmp/ directory that are based on model names becomes problematic on multi-user systems since if one user runs LLM("facebook/opt-125"), then a filelock is created at /tmp/facebook-opt-125m.lock and left there. This is fine if a single user is doing this, but as soon as another user runs LLM("facebook/opt-125"), then vLLM will try to access the filelock that another user made, which they will not have permission for, triggering:

PermissionError: [Errno 13] Permission denied: '/tmp/facebook-opt-125m.lock'

This PR attempts to resolve this by using a user's local ~/.cache/vllm/locks directory to create and access locks, preventing file permissions conflicts with other users.

Addresses: #2179 #2232 #2675

[zhuohan123]

LGTM! Left a small comment.

[mgoin]

Thanks for the review @zhuohan123! Regarding the location and XDG_CACHE_HOME, I took the precedent from the Usage Stats PR #2852 since they use the XDG_CONFIG_HOME to control ~/.config

I think the failing lora test is flaky but I don't seem able to rerun the job