release-21.2: backup: use correct Context in restore workers

[blathers-crl]

Backport 1/1 commits from #74905 on behalf of @dt.

/cc @cockroachdb/release

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Previously some of the workers, which are called by ctxgroup goroutines,
were using RestoreDataProcessor.Ctx, instead of the child context that
the group created, which, critically, is cancelled if any group task
fails. This could mean one worker in the group fails and stops draining
a channel and returns an error to the group, which cancels its context,
but another worker trying to write to that channel hangs if it is not
checking the passed, now cancelled context.

Release note (bug fix): fix a case where a RESTORE job could hang if it encountered an error when ingesting restored data.

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Release justification: bug fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is 

[dt]

Do we think we should backport this? I was mostly just testing a blathers change and needed a tiny clean patch to test.... but I suppose it is bug to fix?