Resolve most shellcheck warnings.

[jeremyje]

What type of PR is this?
/kind cleanup

What this PR does / why we need it:
Addresses most of the shellcheck errors that would come up with kubernetes-sig/csi-release-tools.

Which issue(s) this PR fixes:
#585 partial

Special notes for your reviewer:

Does this PR introduce a user-facing change?:
-->

NONE

[k8s-ci-robot]

Hi @jeremyje. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[msau42]

/ok-to-test

[msau42]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jeremyje, msau42

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