shell: fix pass overlays to devshell as well

[blaggacao]

see also: https://github.com/numtide/flake-utils/pull/18/files#diff-79c5531512c8fa0e4522e5d82d96fc2d93b75040e0a197cb0a94c7cb0b2c19b3R178-R187

[blaggacao]

Apparently it yet doesn't do what I intended.

[blaggacao]

I'm not sure if we generally want to do this since in casu it only affects nixflk's own shell.nix. The reason I wanted to attempt it is to pass along nixUnstable, so that we have better control over latest and greatest nix features. (I'm not sure how much stable branch lags behind, yesterday it was just a few days f a release).

While dogfooding the overlays is probably right, I'm not entirely sure about the overrides.

On the other hand, thinking twice, maybe passing overlays to specifically nixflk/shell.nix maybe is overkill and should be avoided.

So I'll retract here and adapt my flake-utils PR accordingly with a hint for reasoning.

[blaggacao]

I correct: we need it upstream in flake-utils for a potentially more generic use case, but we don't need it here for the current nixflk/shell.nix. If flake-utils should be adapted, we could get it for free, then.