dune doesn't reset DUNE_CACHE_ROOT inside cram tests

[davesnx]

Opening this issue to see if there's an alternative I'm missing but running dune inside a cram tests makes warnings/hints appear about cache (and most probably any global setup for dune)

If DUNE_CACHE_ROOT is /path/that/does/not/exist/temp this output pops out:

   $ dune build @melange
+  Warning: Cache directories could not be created: Read-only file system;
+  disabling cache
+  Hint: Make sure the directory /path/that/does/not/exist/temp can be created

Reference issue from reasonml/reason-react#870 (comment)

Is there any cache, or set of env variables that should be set on cram tests by users or by default?

[davesnx]

I found this #6138 to be related to the dune's test itself, and @emillon didn't want to disable cache inside cram tests for dune itself.