Fixup various issues with the nix.conf / nix.custom.conf split #2624

Job	Run time
Lints	2m 52s
build-aarch64-darwin / Build aarch64 Darwin (static)	7m 23s
build-aarch64-linux / Build aarch64 Linux (static)	7m 43s
build-x86_64-darwin / Build x86_64 Darwin (static)	13m 20s
build-x86_64-linux / Build x86_64 Linux (static)	5m 47s
Run x86_64 Linux (No init${{ ((matrix.determinate && ', --determinate') \|\| '') }})	0s
Run x86_64 Linux${{ ((matrix.determinate && ' (--determinate)') \|\| '') }}	0s
Run x86_64 Linux release check matrix	0s
Run x86_64 Linux release checks	0s
Run aarch64 Darwin (--determinate)	1m 58s
Run aarch64 Darwin	2m 3s
Run aarch64 Linux${{ ((matrix.determinate && ' (--determinate)') \|\| '') }}	0s
Run x86_64 Darwin (--determinate)	5m 15s
Run x86_64 Darwin	3m 57s
	50m 18s

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