backupccl: don't include tenants in non-cluster backups

[dt]

Release note (bug fix): System tenant backups of individual tables and databases no longer include tenants as well.

[cockroach-teamcity]

This change is 

[stevendanna]

Thanks for adding the test as well. I think this looks good as a backportable fix.

[dt]

TFTR!

bors r+

[craig]

Build succeeded:

• GitHub 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 ddd8761 to blathers/backport-release-21.1-72871: 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 21.1.x failed. See errors above.

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