Fixup various issues with the nix.conf / nix.custom.conf split #2623

Job	Run time
Lints	3m 16s
build-aarch64-darwin / Build aarch64 Darwin (static)	8m 11s
build-aarch64-linux / Build aarch64 Linux (static)	7m 34s
build-x86_64-darwin / Build x86_64 Darwin (static)	11m 52s
build-x86_64-linux / Build x86_64 Linux (static)	5m 46s
Run x86_64 Linux${{ ((matrix.determinate && ' (--determinate)') \|\| '') }}	0s
Run x86_64 Linux (No init${{ ((matrix.determinate && ', --determinate') \|\| '') }})	0s
Run x86_64 Linux release check matrix	0s
Run x86_64 Linux release checks	0s
Run aarch64 Linux${{ ((matrix.determinate && ' (--determinate)') \|\| '') }}	0s
Run aarch64 Darwin (--determinate)	2m 6s
Run aarch64 Darwin	2m 2s
Run x86_64 Darwin (--determinate)	3m 22s
Run x86_64 Darwin	5m 12s
	49m 21s

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