Add a ‘--repair’ flag to nix-instantiate

This allows repairing corrupted derivations and other source files.

src/libexpr/eval.hh

@@ -95,6 +95,10 @@ public:
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName,
         sSystem, sOverrides;
 
+    /* If set, force copying files to the Nix store even if they
+       already exist there. */
+    bool repair;
+
 private:
     SrcToStore srcToStore;