nixos: fix kconfig string

Fix a typo on nix kconfig. The automated testing didn't pick this up
because we never run make menuconfig. We just run defconfigs.

Fixes 3b68685 ("mirror: add Nix binary cache mirroring support")
Signed-off-by: Luis Chamberlain <[email protected]>

Author: mcgrof

kconfigs/Kconfig.nixos

@@ -110,7 +110,7 @@ config NIXOS_USE_LOCAL_MIRROR
 config NIXOS_MIRROR_URL
 	string "Nix binary cache mirror URL"
 	output yaml
-	default $(shell, scripts/check_nix_mirror.sh NIX_CACHE_MIRROR_URL)
+	default "$(shell, scripts/check_nix_mirror.sh NIX_CACHE_MIRROR_URL)"
 	depends on NIXOS_USE_LOCAL_MIRROR
 	help
 	  The URL of the local Nix binary cache mirror. This is