CreateOrMergeNixConfig: fix expected nix.conf mode to be 644, not 664 #2567

Job	Run time
Lints	2m 41s
build-aarch64-darwin / Build aarch64 Darwin	7m 55s
build-x86_64-linux / Build x86_64 Linux (static)	7m 12s
build-aarch64-linux / Build aarch64 Linux (static)	10m 22s
build-x86_64-darwin / Build x86_64 Darwin	12m 18s
Run x86_64 Linux (--determinate)	1m 13s
Run x86_64 Linux (No init, --determinate)	1m 46s
Run x86_64 Linux release check matrix	0s
Run x86_64 Linux	1m 3s
Run x86_64 Linux (No init)	1m 33s
Run x86_64 Linux release checks	0s
Run aarch64 Darwin (--determinate)	2m 6s
Run aarch64 Darwin	2m 7s
Run aarch64 Linux (--determinate)	1m 24s
Run aarch64 Linux	1m 14s
Run x86_64 Darwin (--determinate)	3m 40s
Run x86_64 Darwin	3m 34s
	1h 0m 8s

Provide feedback