Remove arch specification so mise can be installed on arm64 linux

What does this MR do?

When building the Dockerfile image locally on MacOS, mise is failing to install because we specify arch=amd64. By removing that it safely build in both arm64 and amd64 architectures.

Related issues

• Closes #1167 (closed)
• Related to: #1166 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gitlab-org/software-delivery/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected across all expected permutations.
  • Check that the changes work across upgrades.
  • Documentation created/updated in the same MR if applicable