Read-only file may be left in writable state

[sds83uk]

I added a comment to issue #167 to a few days ago, unfortunately a problem with my account meant that I couldn't highlight this further and the comment would not have been displayed on the issue at the time the "patch released" label was added and the issue closed.

I believe that when --fix is used a read-only file may be left writable if:

1. The file is locked by other process.
2. The file contains the CVE-2021-45105 vulnerability (requiring an upgrade)
3. The file is backed up (after setWritable) and later restored
4. The backup file cannot be truncated

NB: With release 2.5.0, similar edge cases may exist when attempting to --restore a read-only targetFile from a backup.

[xeraph]

@sds83uk Would you test v2.6.2? 7427988