Fix the test for recursive calling of Nimble tasks

The test case for recursive calling of Nimble tasks was not working
properly on Windows so far.

Related to #680

tests/recursive/recursive.nimble

@@ -9,13 +9,18 @@ license       = "BSD"
 
 requires "nim >= 0.12.1"
 
+when defined(windows):
+  let callNimble = "..\\..\\src\\nimble.exe"
+else:
+  let callNimble = "../../src/nimble"
+
 task recurse, "Level 1":
   echo 1
-  exec "../../src/nimble recurse2"
+  exec callNimble & " recurse2"
 
 task recurse2, "Level 2":
   echo 2
-  exec "../../src/nimble recurse3"
+  exec callNimble & " recurse3"
 
 task recurse3, "Level 3":
   echo 3