Update -shell-image to a multi-arch version

[mattmoor]

Changes

Update the shell image to be multi-arch (and pick up any updates). This also updates the image reference, which was incompatible with the comment (wrt root vs. nonroot).

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
• Commit messages follow commit message best practices
• Release notes block has been filled in or deleted (only if no user facing changes)

See the contribution guide for more details.

Double check this list of stuff that's easy to miss:

• If you are adding a new binary/image to the cmd dir, please update
  the release Task to build and release this image.

Reviewer Notes

If API changes are included, additive changes must be approved by at least two OWNERS and backwards incompatible changes must be approved by more than 50% of the OWNERS, and they must first be added in a backwards compatible way.

Release Notes

Use a multi-arch shell-image

cc @vdemeester @imjasonh

[imjasonh]

/kind feature

/lgtm

[tekton-robot]

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