Update nix dependency to address CVE-2021-45707

[grahambrereton-form3]

Updating nix to latest required updating rustc. This prompted a change to use stable instead of the old nightly channel build from 2021.

I think the switch from nightly is justified:

• Full relro is the default behaviour on Linux and x64, which is the only possible current target for this repo as far as I can tell, so the experimental build flag is not required.
• Vec::into_raw_parts is experimental, but not necessary and you can work around its absence.
• LinkedList::drain_filter is not necessary, given there doesn't appear to be any benefit to using a linked list in this case. A simple Vec suffices for the stack-like usage, and it provides retain to remove all non-matching items.

File built on stable with no -Z relro-level=full option:

❯ checksec --file=./toda 
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH	Symbols		FORTIFY	Fortified	Fortifiable  FILE
Full RELRO      No canary found   NX enabled    PIE enabled     No RPATH   No RUNPATH   19690 Symbols     No	0		10	./toda

[chaotic-prow]

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