ci: fix image signing on main

[skl]

The good news is #1327 reduced image build and publish time from 45m to 5m 🚀

The bad news is it also broke image signing on main (example):

could not parse reference: otel/opentelemetry-ebpf-k8s-cache:main@Name

This PR fixes the multi-arch manifest digest parsing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.27%. Comparing base (0bf4f8f) to head (7d14ac5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1332       +/-   ##
===========================================
+ Coverage   19.63%   44.27%   +24.63%     
===========================================
  Files         239      303       +64     
  Lines       27971    27689      -282     
===========================================
+ Hits         5493    12259     +6766     
+ Misses      21831    14701     -7130     
- Partials      647      729       +82     

Flag	Coverage Δ
integration-test	?
integration-test-arm	?
integration-test-vm-x86_64-5.15.152	?
integration-test-vm-x86_64-6.10.6	?
k8s-integration-test	?
oats-test	?
unittests	44.27% <ø> (?)

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