ci_set_matrix: build the matrix output explicitly

"ports" was derived from the keys of the dict that "split_ports" was
then inserted into, so the two lines only worked in that order. Build
the output from the matrix entries in one place instead. "split_ports"
stays in the output even when empty, because build.yml indexes it.

Author: lynt-smitka

tools/ci_set_matrix.py

@@ -284,12 +284,18 @@ def get_settings(board):
                 port_to_boards_to_build[name] = boards[index::parts]
                 split_ports[name] = port
 
+    # The matrix entries plus what build.yml needs to drive them. "split_ports" is always
+    # present, empty when nothing was split, because build.yml indexes it.
+    output = {}
     if port_to_boards_to_build:
-        port_to_boards_to_build["ports"] = sorted(list(port_to_boards_to_build.keys()))
-        port_to_boards_to_build["split_ports"] = split_ports
+        output = {
+            **port_to_boards_to_build,
+            "ports": sorted(port_to_boards_to_build),
+            "split_ports": split_ports,
+        }
 
     # Set the step outputs
-    set_output("ports", json.dumps(port_to_boards_to_build))
+    set_output("ports", json.dumps(output))
 
 
 def set_docs(run: bool):