backupccl: add missing ctx cancel check

[adityamaru]

In #111159 we deduced from the stacks a
situation in which the goroutine draining spanCh had exited due to a context cancelation but the
writer was not listening for a ctx cancelation.
This manifests as a stuck restore when using
the non-default make simple import spans implementation.

Fixes: #111159
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[msbutler]

This is, in theory, unit test able. but this is also dead, non default code. And we have bigger things on our plate.

[adityamaru]

This is, in theory, unit test able. but this is also dead, non default code. And we have bigger things on our plate.

the exact thought that went through my head. If it makes sense to you/rui I think I feel pretty good just merging it for now.

[adityamaru]

TFTR!

bors r=msbutler

[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 6fabaf8 to blathers/backport-release-22.2-111231: 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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_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}