fix: ubi image build should support multi arch builds

[TomerFi]

Following stacklok/toolhive-registry-server#123.

Using Mac M2 pro, bulding ARM:

task build-operator-image-ubi
task build-proxyrunner-image-ubi

Building AMD:

PLATFORM=linux/amd64 task build-operator-image-ubi
PLATFORM=linux/amd64 task build-proxyrunner-image-ubi

cc @dmjb @dmartinol

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.21%. Comparing base (b67e9a2) to head (15fe2e4).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2655   +/-   ##
=======================================
  Coverage   55.21%   55.21%           
=======================================
  Files         315      315           
  Lines       30123    30123           
=======================================
+ Hits        16632    16633    +1     
  Misses      12047    12047           
+ Partials     1444     1443    -1     

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[jhrozek]

works fine