[bugfix] ensure the 'Closing' flag doesn't get cached

[NyaaaWhatsUpDoc]

Description

Ensures the ephemeral poll.Closing flag isn't cached. This is only expected to be set on a singular instance of polls!

closes #2437

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.