fix(erofs): remove tmp file on rename error

[austinvazquez]

Follow-up to #212

Revert the write to tmp, file sync + rename bits to simplify creating the vmdk files. I dove through the annals and found the review comment which sent my agent down this path. Let's remove it to simplify. This bit of code is not really executed concurrently for the current use case.

5. .tmp suffix is not randomised
   DumpVMDKDescriptorToFile uses a fixed path + ".tmp" suffix. Two concurrent callers (unlikely but possible given transformMounts holds no lock) would silently clobber each other's .tmp file. A safer approach is os.CreateTemp(descDir, "merged_fs.vmdk.*.tmp") + rename.

This is low-risk in practice (single-threaded per container lifecycle) but worth noting for robustness.

[Copilot]

Pull request overview

This PR is a follow-up to #212 that ensures DumpVMDKDescriptorToFile best-effort cleans up its temporary merged_fs.vmdk.*.tmp file when the final os.Rename step fails, preventing stray temp files from persisting on rename errors.

Changes:

• Convert DumpVMDKDescriptorToFile to use a named return error so a deferred cleanup can run on any returned error.
• Add a deferred best-effort os.Remove(tmpName) cleanup path when the function exits with an error (including rename failures).

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[austinvazquez]

@hsiangkao , sorry for the churn. I decided to dive into where my agent brought this tmp file + rename from and found it was just a minor comment from itself during its own review. Let's just revert it back to simply write the file with content. We can keep the unit tests though. WDYT?