chore(pkg,docs): use a working mirror for archlinux arm64.

[FedeDP]

What type of PR is this?

/kind cleanup
/kind documentation

Any specific area of the project related to this PR?

/area pkg
/area docs

What this PR does / why we need it:

See falcosecurity/kernel-crawler#196.
Also, updated builders documentation after recent changes.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[FedeDP]

/hold
The mirror went down right after i opened the kernel-crawler + this PR :/ https://github.com/falcosecurity/kernel-crawler/actions/runs/8566017164/job/23475016449

[poiana]

Issues go stale after 90d of inactivity.

Mark the issue as fresh with /remove-lifecycle stale.

Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Provide feedback via https://github.com/falcosecurity/community.

/lifecycle stale

[FedeDP]

/unhold
the mirror is back up!
/cc @LucaGuerra

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: FedeDP, LucaGuerra

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [FedeDP,LucaGuerra]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment