dnsapi/dns_gandi_livedns.sh: save Gandi LiveDNS variables with prefix 'SAVED_'

[pini-gh]

Fixes #6081.

The LiveDNS authentication variables are now saved and retrieved using respectively _saveaccountconf_mutable and _readaccountconf_mutable functions.

This fixes an issue with tools such as nginx-proxy/acme-companion where authentication settings are done using environment variables only: new values for the authentication token were ignored and the saved one was used instead.

[github-actions]

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[Neilpang]

it's easy to fix 6081.
just use _saveaccountconf_mutable instead of _saveaccountconf

And use _readaccountconf_mutable at the beginning .

[pini-gh]

Thank you, I wasn't aware of this _mutable flavor. I fixed the PR.