autotest: make scripts directory relative to CWD

This is where the library is expecting to find the scripts directory.

The existing setup assumes that autotest has been run from the root
directory - that's not always the case.  For example, the autotest
server starts everything one-up from the root directory.

Tools/autotest/apmrover2.py

@@ -4689,7 +4689,7 @@ Brakes have negligible effect (with=%0.2fm without=%0.2fm delta=%0.2fm)
         return os.path.join(self.rootdir(), "libraries", "AP_Scripting", "examples", scriptname)
 
     def installed_script_path(self, scriptname):
-        return os.path.join(self.rootdir(), "scripts", scriptname)
+        return os.path.join("scripts", scriptname)
 
     def install_example_script(self, scriptname):
         source = self.script_source_path(scriptname)