builtins: don't panic on placeholders in bounded staleness function

[rafiss]

fixes #86243

Release note (bug fix): Fixed a crash/panic that could occur if
placeholder arguments were used with the with_min_timestamp or
with_max_staleness functions.

Release justification: Fix a crash caused by a panic.

[cockroach-teamcity]

This change is 

[rafiss]

tftr!

bors r+

[craig]

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[rafiss]

bors r+

[craig]

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[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 34f6bf9 to blathers/backport-release-21.2-86605: 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 21.2.x failed. See errors above.

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error creating merge commit from 34f6bf9 to blathers/backport-release-22.1-86605: 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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