datetime: obey the evalengine's environment time

[vmg]

Description

The evaluation environment in the evalengine sets a frozen time.Time value upon creation, to ensure that all time-related APIs in the engine are evaluated consistently, following MySQL's behavior.

However, there are APIs in the datetime package that call time.Now() directly, particularly when converting from a time value to a datetime value. This is not correct! These APIs must use the frozen time set in the evalengine. This PR updates the APIs to take a time input when necessary.

cc @dbussink

Related Issue(s)

Found when working on #14310, follow up to #14351

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[vitess-bot]

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[vmg]

@dbussink: the tests we wrote yesterday break if they're not run yesterday 😅