test: Use arm64 runners for arm64 container integration tests

[tippmar-nr]

Updates container integration test workflow to split amd64 and arm64 tests to two separate jobs, each running on an appropriate Github runner. Also removed the use of Qemu in the workflow, as it's no longer required.

Tests were updated to add an "Architecture" trait so it's easy to divide them between the two jobs.

Local execution is expected to remain unchanged, still needing to leverage Qemu. The magic is in the BUILD_ARCH environment variable -- when present, we use the specified architecture to build the container test app; when not present (i.e., running local), we hard-code it to amd64.

An example workflow run is at https://github.com/newrelic/newrelic-dotnet-agent/actions/runs/12917552676/job/36024160825

[chynesNR]

Nice work!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.80%. Comparing base (f622a40) to head (d8997b2).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2959   +/-   ##
=======================================
  Coverage   81.80%   81.80%           
=======================================
  Files         470      470           
  Lines       29953    29953           
  Branches     3330     3330           
=======================================
  Hits        24504    24504           
  Misses       4657     4657           
  Partials      792      792           

Flag	Coverage Δ
Agent	82.76% <ø> (ø)
Profiler	73.13% <ø> (ø)

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