workaround for allocations when broadcasting equations

[ranocha]

See #1618

Together with #1625, this should hopefully fix the current CI issues.

Closes #1618

[github-actions]

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[JoshuaLampert]

Let's hope JuliaLang/julia#51118 will be fixed...

[ranocha]

I hope so, too, but I'm not very optimistic, to be honest. The issue is solved in Julia v1.10 - the next stable release. Thus, I guess there is not much pressure to fix this specific issue. Of course, for us it's very important since there are much more severe issues with the current pre-release version of Julia v1.10, e.g., JuliaLang/julia#50985, JuliaLang/julia#50562

[ranocha]

I'll merge this now since non-coverage CI passes - except the test fixed in #1625