Use nixos stable for nix flake

[benalleng]

This moves our nix flake from unstable to the stable channel.

Closes #1187

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[coveralls]

Pull Request Test Coverage Report for Build 19514086147

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.578%

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Totals
Change from base Build 18946545260:	0.0%
Covered Lines:	8988
Relevant Lines:	10754

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💛 - Coveralls

[nothingmuch]

utACK, i will test only tomorrow as it's late and i'm working on something else

note that 25.11 should be out soon, so it might be wiser to stick with unstable until then and effectively treat 25.11 as freezing that instead of downgrade and upgrade in case that breaks anyone's setup, but in practice i don't think there will be much of an issue so i'm in favor of merging. such breakage would be much more sensitive to changes in the rust toolchain via rust overlay flake, especially because until a fairly recent flake.lock upgrade i think we were on unstable but because it was stale it was equivalent to 24.11 or so. anyway all this just goes to say if something does break then we have an easy out via 25.11

[arminsabouri]

Is this good to merge?

[DanGould]

i'm in favor of merging

-- @nothingmuch