tf: fix benchmarking standalone apm to use x86_64 AMI by default

[carsonip]

Motivation/summary

Fix a regression where benchmarking standalone apm server does not find a matching x86_64 AMI for c6i instance types.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

Fixes #15604

[mergify]

This pull request does not have a backport label. Could you fix it @carsonip? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[carsonip]

If https://github.com/elastic/apm-server/actions/runs/13248610640 passes, this PR should be good to merge.

[carsonip]

terraform apply works. PR is good to merge.