dune doesn't reset DUNE_CACHE_ROOT inside cram tests

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 https://github.com/reasonml/reason-react/issues/870#issuecomment-2681483581

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

Start by reproducing the warning in a cram test that runs dune build @melange with DUNE_CACHE_ROOT set to the nonexistent path shown. Read the cram-test environment setup and determine whether cache-related variables should be reset or configured; done means the test no longer emits the cache warning without breaking cache behavior.