ci: target the macOS runner by label name macos (not macos:host)

In act_runner a label is `<name>:<backend>`; `:host` is the execution-
backend schema (run on host, no container), not part of the label. The
runner registered as `macos:host` therefore has the label `macos`, which
is what runs-on must reference.

.gitea/workflows/macos-probe.yaml

@@ -19,7 +19,10 @@ on:
 
 jobs:
   probe:
-    runs-on: "macos:host"
+    # Label NAME only. The runner was registered as `macos:host`, but `:host` is
+    # act_runner's execution-backend schema (run on the host, no container), not
+    # part of the label — so the label is just `macos`.
+    runs-on: macos
     steps:
       - name: identity, SDK, toolchains
         run: |