Avoid msan reports on globals destructors

Somehow tests load these libs twice, and construct/destruct globals twice.
Can be preproduced even without sanitizers with logging from GlobalModel::~GlobalModel.

themodel does not need to be visible. If it's static each instance will have own a copy.
_mjMap does not need to use string and avoid lifecycle issues at all.

PiperOrigin-RevId: 454678814
Change-Id: I8fca5e1f06cbc2b413beabcece1219445482c0ed

src/xml/xml_util.h

@@ -76,7 +76,7 @@ class mjXSchema {
 
 // key(string) : value(int) map
 struct _mjMap {
-  std::string key;
+  const char* key;
   int value;
 };
 typedef struct _mjMap mjMap;