CI: Update matrix.py so all code is contained in functions.

Refactor matrix.py so all the logic is contained inside either
helper functions or a new main function. Run the new main function
by default. This lets other GitHub actions use functions in the
python code without generating the whole matrix.

.github/matrix.py

@@ -15,12 +15,6 @@ import sys
 import urllib.request
 from os import environ
 
-if len(sys.argv) == 2:
-    ref_name = sys.argv[1]
-else:
-    print("Usage: {} <ref_name>".format(sys.argv[0]), file=sys.stderr)
-    sys.exit(1)
-
 #
 # this CI is used for both development and stable branches of HAProxy
 #
@@ -29,8 +23,6 @@ else:
 #   "haproxy-" - stable branches
 #   otherwise  - development branch (i.e. "latest" ssl variants, "latest" github images)
 #
-print("Generating matrix for branch '{}'.".format(ref_name))
-
 
 def clean_ssl(ssl):
     return ssl.replace("_VERSION", "").lower()
@@ -101,6 +93,8 @@ def get_asan_flags(cc):
         'CPU_CFLAGS.generic="-O1"',
     ]
 
+def main(ref_name):
+    print("Generating matrix for branch '{}'.".format(ref_name))
 
     matrix = []
 
@@ -258,3 +252,11 @@ print(json.dumps(matrix, indent=4, sort_keys=True))
     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)
+
+if __name__ == "__main__":
+    if len(sys.argv) == 2:
+        ref_name = sys.argv[1]
+        main(ref_name)
+    else:
+        print("Usage: {} <ref_name>".format(sys.argv[0]), file=sys.stderr)
+        sys.exit(1)