[CI] Set PYTHONIOENCODING to utf-8 to make mx happy

Starting with mx 7.24.0, mx checks if the stdout encoding is set to
unicode.

See
graalvm/mx@b6625a4

.github/workflows/buildJDK.yml

@@ -18,6 +18,7 @@ env:
   # This ought to be a valid version parseable by Version.parse()
   MANDREL_VERSION: 10.9.8.7-dev
   MX_PYTHON: python3
+  PYTHONIOENCODING: utf-8
 
 # The following aims to reduce CI CPU cycles by:
 # 1. Cancelling any previous builds of this PR when pushing new changes to it