Fix posix ApplyEnvironmentModifications for repeated modifications

This behavior is correctly applied on Windows but on posix systems would fail to apply duplicate
entries that should be additive, such as multiple prepends. Previously only the last modification
would win.

renderdoc/os/posix/posix_process.cpp

@@ -415,7 +415,7 @@ void ApplyEnvironmentModifications(rdcarray<EnvironmentModification> &modificati
   {
     EnvironmentModification &m = modifications[i];
 
-    rdcstr value = currentEnv[m.name.c_str()];
+    rdcstr &value = currentEnv[m.name.c_str()];
 
     ApplySingleEnvMod(m, value);