FetchAndUnpackNix: remove destination if it already exists #2492

Job	Run time
Lints	2m 32s
build-aarch64-darwin / Build aarch64 Darwin	7m 29s
build-aarch64-linux / Build aarch64 Linux (static)	9m 50s
build-x86_64-darwin / Build x86_64 Darwin	11m 31s
build-x86_64-linux / Build x86_64 Linux (static)	6m 38s
Run aarch64 Darwin (--determinate)	2m 20s
Run aarch64 Darwin	2m 26s
Run x86_64 Linux (--determinate)	2m 0s
Run x86_64 Linux release checks	0s
Run x86_64 Linux (No init, --determinate)	1m 42s
Run x86_64 Linux	1m 25s
Run x86_64 Linux (No init)	1m 21s
Run aarch64 Linux (--determinate)	1m 38s
Run aarch64 Linux	1m 16s
Run x86_64 Darwin (--determinate)	5m 5s
Run x86_64 Darwin	5m 11s
	1h 2m 24s

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