fix(ci): target macOS runner explicitly for iOS builds

The 'self-hosted' label alone matched the Linux runner which cannot
build iOS. Change to [self-hosted, macos] so only the Mac builder
on-prem picks up iOS build jobs.

.gitea/workflows/ci.yml

@@ -201,7 +201,7 @@ jobs:
   # if Apple signing secrets are not configured.
   build-ios:
     name: Build iOS
-    runs-on: self-hosted
+    runs-on: [self-hosted, macos]
     needs: [test]
     steps:
       - name: Checkout

.gitea/workflows/release.yml

@@ -164,7 +164,7 @@ jobs:
   # if Apple signing secrets are not configured.
   build-ios:
     name: Build iOS Release
-    runs-on: self-hosted
+    runs-on: [self-hosted, macos]
     needs: [test]
     steps:
       - name: Checkout