Fix issue 20097

[tchaloupka]

Allow SysTime to be sendable with std.concurrency.send

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20097	normal	SysTime cannot be used with std.concurrency.send

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#7122"

[Geod24]

It would be great if the title of your commit (and PR) included the title of the issue you're fixing.
When one looks at notifications, or use git log, they have no clue what "issue 20097" is, but the title allow to quickly get an idea without opening every single PR / git showing every commit.

Otherwise, LGTM.