Make "${./path} ..." evaluate to a string, not a path

Wacky string coercion semantics caused expressions like

  exec = "${./my-script} params...";

to evaluate to a path (‘/path/my-script params’), because
anti-quotations are desuged to string concatenation:

  exec = ./my-script + " params...";

By constrast, adding a space at the start would yield a string as
expected:

  exec = " ${./my-script} params...";

Now the first example also evaluates to a string.

src/libexpr/nixexpr.hh

@@ -277,8 +277,10 @@ MakeBinOp(OpConcatLists, "++")
 
 struct ExprConcatStrings : Expr
 {
+    bool forceString;
     vector<Expr *> * es;
-    ExprConcatStrings(vector<Expr *> * es) : es(es) { };
+    ExprConcatStrings(bool forceString, vector<Expr *> * es)
+        : forceString(forceString), es(es) { };
     COMMON_METHODS
 };