nix-tools: overwrite $PATH rather than prepending

I don’t see any reason for these scripts to depend on more
environmental state than necessary.

pkgs/nix-tools/darwin-option.sh

@@ -1,7 +1,7 @@
 #! @shell@
 set -e
 set -o pipefail
-export PATH=@path@:$PATH
+export PATH=@path@
 
 evalNix() {
   nix-instantiate --eval --strict "${extraEvalFlags[@]}" -E "with import <darwin> {}; $*" 2>/dev/null

pkgs/nix-tools/darwin-rebuild.sh

@@ -1,8 +1,7 @@
 #! @shell@
 set -e
 set -o pipefail
-export PATH=@path@:$PATH
-
+export PATH=@path@
 
 showSyntax() {
   echo "darwin-rebuild [--help] {edit | switch | activate | build | check | changelog}" >&2