Replace Unicode quotes in user-facing strings by ASCII

Relevant RFC: NixOS/rfcs#4

$ ag -l | xargs sed -i -e "/\"/s/’/'/g;/\"/s/‘/'/g"

src/nix/installables.cc

@@ -221,7 +221,7 @@ std::vector<std::shared_ptr<Installable>> InstallablesCommand::parseInstallables
             result.push_back(std::make_shared<InstallableAttrPath>(*this, s));
 
         else
-            throw UsageError("don't know what to do with argument ‘%s’", s);
+            throw UsageError("don't know what to do with argument '%s'", s);
     }
 
     return result;