fix(treewide): remove unnecessary copying in range for loops

This gets rid of unnecessary copies in range-based-for loops and
local variables, when they are used solely as `const &`.

Also added a fixme comment about a suspicious move out of const,
which might not be intended.

src/libstore/build/drv-output-substitution-goal.cc

@@ -32,7 +32,7 @@ Goal::Co DrvOutputSubstitutionGoal::init()
 
     bool substituterFailed = false;
 
-    for (auto sub : subs) {
+    for (const auto & sub : subs) {
         trace("trying next substituter");
 
         /* The callback of the curl download below can outlive `this` (if