CreateOrMergeNixConfig: fix expected nix.conf mode to be 644, not 664 #2566

Job	Run time
Lints	2m 42s
build-aarch64-linux / Build aarch64 Linux (static)	10m 32s
build-x86_64-darwin / Build x86_64 Darwin	12m 10s
build-aarch64-darwin / Build aarch64 Darwin	8m 16s
build-x86_64-linux / Build x86_64 Linux (static)	7m 17s
Run aarch64 Darwin (--determinate)	2m 2s
Run aarch64 Darwin	2m 8s
Run x86_64 Linux (--determinate)	1m 37s
Run x86_64 Linux (No init, --determinate)	1m 38s
Run x86_64 Linux	1m 29s
Run x86_64 Linux release check matrix	0s
Run x86_64 Linux (No init)	1m 48s
Run x86_64 Linux release checks	0s
Run aarch64 Linux (--determinate)	1m 34s
Run aarch64 Linux	1m 13s
Run x86_64 Darwin (--determinate)	3m 59s
Run x86_64 Darwin	3m 41s
	1h 2m 6s

Provide feedback