[FIX] base: RTLCSS on Windows

RTLCSS is not working on Windows for several reasons:
- the executable is called `rtlcss.cmd`
- rtlcss is not available in the System Path; it is only available in
  the User Path

In this commit, we correct the first issue.

opw-1892952

odoo/addons/base/models/assetsbundle.py

@@ -525,6 +525,8 @@ class AssetsBundle(object):
 
     def run_rtlcss(self, source):
         rtlcss = 'rtlcss'
+        if os.name == 'nt':
+            rtlcss = misc.find_in_path('rtlcss.cmd')
         cmd = [rtlcss, '-']