stdenv: Prevent issues like #4266

Don't preserve hardlinks, and instead use reflinks if they're available.

pkgs/stdenv/generic/setup.sh

@@ -463,7 +463,9 @@ _defaultUnpack() {
     if [ -d "$fn" ]; then
 
         stripHash "$fn"
-        cp -prd --no-preserve=timestamps "$fn" $strippedName
+        # We can't preserve hardlinks because they may have been introduced by
+        # store optimization, which might break things in the build
+        cp -pr --reflink=auto --no-preserve=timestamps "$fn" $strippedName
 
     else