ENH: provide argList::envGlobalPath() static method

- this is identical to either of these solutions:

     * getEnv("FOAM_CASE")
     * stringOps::expand("<case>")

  but with a closer resemblance to argList or Time globalPath(),
  which makes the intent clearer.
  Avoids using raw strings in the caller, which improves compile-time checks.

  Used in situations where a class has no derivation path or other
  access to a time registry or command args.