Fix shellcheck output streams

[YuviGold]

What type of PR is this?

/kind documentation
/kind failing-test

What this PR does / why we need it:

shellcheck errors are printed to stdout by default, hence they need to be redirected
to stderr in order to be well parsed for Junit representation by juLog function.

Which issue(s) this PR fixes:

Fixes #102975

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

Now the generated Junit would have the correct failures in it to be shown later on by Spyglass:

<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
    <testsuite failures="1" assertions="" name="verify" tests="1" errors="1" time="33.3179">
    
    <testcase assertions="1" name="shellcheck" time="33.3179" classname="verify">
    
      <failure type="ScriptError" message="Script Error"><![CDATA[
In ./hack/verify-shellcheck.sh line 25:
BLA=3
^-^ SC2034: BLA appears unused. Verify use (or export if used externally).

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- BLA appears unused. Verify use (o...]]></failure>
  
    <system-err><![CDATA[
In ./hack/verify-shellcheck.sh line 25:
BLA=3
^-^ SC2034: BLA appears unused. Verify use (or export if used externally).

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- BLA appears unused. Verify use (o...]]></system-err>
    </testcase>
  
    </testsuite>
</testsuites>

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[k8s-ci-robot]

Hi @YuviGold. Thanks for your PR.

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[YuviGold]

/sig testing
/cc @BenTheElder

[wzshiming]

/ok-to-test

[YuviGold]

e2e flaky test failed
/retest

[BenTheElder]

hmm: this should be going to stderr still, not stdout?
this is still a diagnostic message, not program output.

[BenTheElder]

thanks :-) 🎉

[BenTheElder]

/lgtm
/approve
🎉 thank you!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BenTheElder, YuviGold

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