packaging: ensure that --python is an absolute path when building on Windows

For whatever reason, even though only python2 is on PATH, passing `python.exe`
causes the later check that it's not py3 to bail out.

contrib/packaging/inno/build.py

@@ -30,6 +30,9 b" if __name__ == '__main__':"
 
     args = parser.parse_args()
 
+    if not os.path.isabs(args.python):
+        raise Exception('--python arg must be an absolute path')
+
     if args.iscc:
         iscc = pathlib.Path(args.iscc)
     else:

contrib/packaging/wix/build.py

@@ -62,6 +62,9 b" if __name__ == '__main__':"
         'version': args.version,
     }
 
+    if not os.path.isabs(args.python):
+        raise Exception('--python arg must be an absolute path')
+
     if args.extra_packages_script:
         kwargs['extra_packages_script'] = args.extra_packages_script
     if args.extra_wxs: