github: add workflow_dispatch trigger to kernel and packages

Allow the Build Kernel and Build all core packages workflows to be
launched manually from the Actions tab. The shared workflow side
detects workflow_dispatch and, for Build Kernel, rebuilds the full
target/subtarget matrix including testing kernel versions, so a
manual run can re-seed the s3 ccache when a queued push run got
displaced from the concurrency queue by the next commit on main.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://github.com/openwrt/openwrt/pull/23283
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.github/workflows/kernel.yml

@@ -19,6 +19,7 @@ on:
       - 'target/linux/**'
     branches-ignore:
       - master
+  workflow_dispatch:
 
 permissions:
   contents: read

.github/workflows/packages.yml

@@ -19,6 +19,7 @@ on:
       - 'toolchain/**'
     branches-ignore:
       - master
+  workflow_dispatch:
 
 permissions:
   contents: read