sql: fix SHOW GRANTS FOR public

[rafiss]

fixes #96948

Release note (bug fix): Fixed the SHOW GRANTS FOR public command so it works correctly. Previously, this would return an error saying that the public role does not exist.

[cockroach-teamcity]

This change is 

[rafiss]

flake is #96978

bors r=ecwall

[craig]

Build failed:

• Bazel Essential CI (Cockroach)

[rafiss]

bors r=ecwall

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from f555196 to blathers/backport-release-22.1-96957: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.1.x failed. See errors above.

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error creating merge commit from f555196 to blathers/backport-release-22.2-96957: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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