release-24.1: backupccl: copy spans before SubtractSpans calls

[blathers-crl]

Backport 1/1 commits from #122819 on behalf of @msbutler.

/cc @cockroachdb/release

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Previously, backup assumed arguments to roachpb.SubtractSpans could be used, but the first arg is modified in place. This patch adds a wrapper to roachpb.SubtractSpans to copy the input spans before use.

Informs #122672
Informs #122734
Informs #122754
Informs #122764

Release note: none

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Release justification: fixes corruption bug

[blathers-crl]

Thanks for opening a backport.

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[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]

abandoning this as we're taking a different approach to prevent empty external file ingestion