FetchAndUnpackNix: remove destination if it already exists (#1319) #2493

Job	Run time
Lints	2m 12s
build-aarch64-darwin / Build aarch64 Darwin	3m 48s
build-aarch64-linux / Build aarch64 Linux (static)	9m 54s
build-x86_64-darwin / Build x86_64 Darwin	10m 56s
build-x86_64-linux / Build x86_64 Linux (static)	3m 21s
Run aarch64 Darwin (--determinate)	2m 20s
Run aarch64 Darwin	2m 24s
Run x86_64 Linux (--determinate)	1m 55s
Run x86_64 Linux (No init, --determinate)	1m 33s
Run x86_64 Linux	1m 47s
Run x86_64 Linux release checks	0s
Run x86_64 Linux (No init)	1m 19s
Run aarch64 Linux (--determinate)	1m 35s
Run aarch64 Linux	1m 19s
Run x86_64 Darwin (--determinate)	4m 13s
Run x86_64 Darwin	5m 18s
	53m 54s

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