Fix heap use after free in progress-bar.cc

Fix some heap-use-after-free in progress-bar.cc

These are somewhat tricky failures here due to temporary variable
creation and string_view

src/libmain/progress-bar.cc

@@ -11,7 +11,7 @@
 
 namespace nix {
 
-static std::string getS(const std::vector<Logger::Field> & fields, size_t n)
+static std::string_view getS(const std::vector<Logger::Field> & fields, size_t n)
 {
     assert(n < fields.size());
     assert(fields[n].type == Logger::Field::tString);