ACS-6450 Dynamically determine ARM64 LibreOffice download link

[mikolajbrzezinski]

This change has been tested on an ARM64 environment and it worked correctly.

[dsibilio]

I'm not usually a fan of Docker builds that fetch stuff remotely and could fail due to changes in an external repository.

Even so, the version-fetching command seems to be specific enough that at least we're not changing the LibreOffice version per se (it's tagged to 6.4.7.2) but only the number after the dash (the package release number, currently -15) which I believe is used in case of very minor re-releases that only involve security patches. This frees us from the burden of manually intervening when the repository is updated ever so slightly.

I'm still wondering how much can change within the repository, it's now been ~7 months since we added support for ARM64 in this repo and the package release number only changed twice. No real version number changed. So I'm hopeful that this automation combined with the stability of the RL repository would save us from further trouble in the future until we decide to move to a newer version of Rocky Linux & LibreOffice altogether.