ci: allow running workflows manually

[skip_ci]

.github/workflows/core.yml

@@ -1,6 +1,8 @@
 name: Core
 
-on: [pull_request]
+on:
+  pull_request:
+  workflow_dispatch:
 
 permissions:
   id-token: write       # for fetching the OIDC token

.github/workflows/legacy.yml

@@ -1,6 +1,8 @@
 name: Legacy
 
-on: [pull_request]
+on:
+  pull_request:
+  workflow_dispatch:
 
 permissions:
   id-token: write       # for fetching the OIDC token