CI: Replace the deprecated `::set-output` command by writing to $GITHUB_OUTPUT in matrix.py

As announced in

https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

the `::set-output` command is deprecated, because processes during the workflow
execution might output untrusted information that might include the
`::set-output` command, thus allowing these untrusted information to hijack the
build.

The replacement is writing to the file indicated by the `$GITHUB_OUTPUT`
environment variable.

.github/matrix.py

@@ -12,6 +12,7 @@ import json
 import sys
 import urllib.request
 import re
+from os import environ
 
 if len(sys.argv) == 2:
     build_type = sys.argv[1]
@@ -208,4 +209,6 @@ for CC in ["clang"]:
 
 print(json.dumps(matrix, indent=4, sort_keys=True))
 
-print("::set-output name=matrix::{}".format(json.dumps({"include": matrix})))
+if environ.get('GITHUB_OUTPUT') is not None:
+    with open(environ.get('GITHUB_OUTPUT'), 'a') as f:
+        print("matrix={}".format(json.dumps({"include": matrix})), file=f)