fix(issue-330): Minimum fix to not always write the config file

[xaviergrandmougin]

Proposed changes

The listen-on-ipv6-by-default script is always trying to modify the /etc/nginx/conf.d/default.conf file, even if the configuration is already correct. This leads to an error in the logs when using a read-only file system.

No attempt to modify /etc/nginx/conf.d/default.conf should be done if the configuration is correct, and this issue is fixed in this short PR.

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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
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[xaviergrandmougin]

I have hereby read the F5 CLA and agree to its terms

[xaviergrandmougin]

The title says "minimum fix", because the grep pattern could be upgraded to something like listen[[:space:]][[:space:]]*\[::]\:8080;.

[alessfg]

Thanks for the fix!