Add a nix.conf option for allowing a symlinked store

src/libstore/globals.hh

@@ -881,6 +881,19 @@ public:
 
     Setting<std::string> flakeRegistry{this, "https://github.com/NixOS/flake-registry/raw/master/flake-registry.json", "flake-registry",
         "Path or URI of the global flake registry."};
+
+    Setting<bool> ignoreSymlinkStore{
+        this, false, "ignore-symlink-store",
+        R"(
+	  If set to `true`, Nix will stop complaining if the store directory
+	  (typically /nix/store) contains symlink components.
+
+	  This risks making some builds "impure" because builders sometimes
+	  "canonicalise" paths by resolving all symlink components. Problems
+          occur if those builds are then deployed to machines where /nix/store
+          resolves to a different location from that of the build machine. You
+          can enable this setting if you are sure you're not going to do that.
+        )"};
 };