(env
 (_
  (env-vars
   (DUNE_FOO -principal))))

(executable
 (flags %{env:DUNE_FOO=-BREAK})
 (name foo))

(alias
 (name default)
 (action
  (progn
   (echo "var visible from dune: %{env:DUNE_FOO=absent}")
   (run ./foo.exe))))