Fix second part of Issue 21722 - toString(sink, string format) does not work with non-"%s" strings

[berni44]

It took me a whole month to understand a comment from @Geod24 in #7870. Sorry for being so ignorant. This is essentially the same fix, but with -preview=in, so I did not create a new issue; hope this is correct - the testers currently don't check with -preview=in as far as I know. I did that locally though. (And I still based on master, due to the heavy changes to std.format in the last weeks; again I hope that's ok.)

Finally, I wonder, why there is no customPutWriterFormatString. Does anybody know a reason? Or was it just forgotten?

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21722	major	toString(sink, string format) does not work with non-"%s" strings

Testing this PR locally

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

dub run digger -- build "master + phobos#7966"