Save release artifacts only after building from source

[pierDipi]

save_release_artifacts assumes that we have built
artifacts from source, I want to run tests without building
from source code.

SKIP_INITIALIZE is the variable used to skip building
the project from source and bootstrapping a cluster.

/cc @aliok @matzew

[codecov]

Codecov Report

Merging #1308 (5978b31) into main (d24282d) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #1308      +/-   ##
============================================
- Coverage     75.67%   75.64%   -0.04%     
+ Complexity      460      459       -1     
============================================
  Files            87       87              
  Lines          3059     3059              
  Branches        136      136              
============================================
- Hits           2315     2314       -1     
- Misses          580      581       +1     
  Partials        164      164              

Flag	Coverage Δ
java-unittests	79.98% <ø> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...dispatcher/impl/consumer/BaseConsumerVerticle.java	89.28% <0.00%> (-3.58%)	⬇️

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[matzew]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matzew, pierDipi

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[matzew]

/test pull-knative-sandbox-eventing-kafka-broker-build-tests

[pierDipi]

/cherry-pick release-0.26

[knative-prow-robot]

@pierDipi: new pull request created: #1453

In response to this:

/cherry-pick release-0.26

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