More fixes for Haiku (#13774)

* osproc: move fork-based code path under the when conditional

* osproc: avoid using the environ global on Haiku

* osenv: import environ from stdlib.h on Haiku

Haiku's environ is declared in `<stdlib.h>` by default, differing from
POSIX and/or Linux. Import it from there to avoid collision with anyone
importing `<stdlib.h>` from Nim.

lib/pure/includes/osenv.nim

@@ -81,6 +81,8 @@ else:
     # at runtime.
     proc NSGetEnviron(): ptr cstringArray {.
       importc: "_NSGetEnviron", header: "<crt_externs.h>".}
+  elif defined(haiku):
+    var gEnv {.importc: "environ", header: "<stdlib.h>".}: cstringArray
   else:
     var gEnv {.importc: "environ".}: cstringArray