ci: make sure BUILDPLATFORM is set in github actions

[Billy99]

#121 removed the BUILDPLATFORM initialization from the Containerfiles.* (ARG BUILDPLATFORM=linux/amd64 -> ARG BUILDPLATFORM). The bpfman-operator Makefile sets that ARG manually, but it wasn't being set in the github actions. This PR adds BUILDPLATFORM to build-arg parameter passed to the docker github action.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 27.61%. Comparing base (8af36b9) to head (aa803a3).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #136   +/-   ##
=======================================
  Coverage   27.61%   27.61%           
=======================================
  Files          81       81           
  Lines        6999     6999           
=======================================
  Hits         1933     1933           
  Misses       4877     4877           
  Partials      189      189           

Flag	Coverage Δ
unittests	27.61% <ø> (ø)

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[anfredette]

LGTM