ci: use writable config locks for offline test containers

[cuichenx]

Problem and change

After #6162 moved configuration locks into HF_HOME, the unprivileged GB200 test container cannot create them in the shared offline model cache. This blocks main and #6414 before model loading. Main failure and bump failure both report the same lock-file permission error.

For functional jobs mounting the offline cache, pass the existing MEGATRON_CONFIG_LOCK_DIR override to the writable container cache. Every rank in these single-container jobs shares that directory. The shared model cache remains offline; the library default and unit-test configuration are unchanged. This preserves #6162's shared-directory default for multi-node users.

This is a base CI integration failure introduced by Bridge #6162, independent of the MCore bump. No upstream Megatron-LM PR causes it; the MCore cause taxonomy does not apply.

Validation

• Regression test executes the actual container-start shell and checks Docker arguments for mounted functional and unmounted unit jobs: 1 failed / 7 passed before; 8 passed after.
• Real safe_load_config_with_retry, AutoConfig, and FileLock with an offline local fixture and effective UID 1000: unwritable cache reproduces the permission failure; a writable lock override succeeds.
• All-file pre-commit and git diff --check pass. Independent review found no blocking findings.
• Requested full L0/L1/L2 CI because the action is shared across the functional matrix. Local evidence does not prove the GPU suite; current-head human review and CI are required before merge.

[github-actions]

Automatic Claude reviews have been retired. To request a pull-request review, post a comment containing:

/review

Add model=claude to use a Claude reviewer (the default is model=codex). mode=light|strict selects the review depth; for example, /review model=claude mode=strict. Comment /review help for all options.