Nim: Fix building with nimble

Do not expect nimble to be in PATH, but assume it is
located where nim resides. Use kit's information of
nim to construct the nimble path and add the path of
nim explicitly to the build environment as nimble
uses it.
Also fixes running nimble test.

Change-Id: If7be425f7b811486afe39fc1618709dbb2f75ac9
Reviewed-by: hjk <hjk@qt.io>

src/plugins/nim/project/nimbuildsystem.h

@@ -86,6 +86,9 @@ public:
 
     void triggerParsing() override;
 
+    Utils::FilePath nimPathFromKit() const;
+    QString defaultNimble() const;
+
 protected:
     void loadSettings();
     void saveSettings();