CI: kernel: generate ccache cache on kernel push

To actually use ccache cache on kernel test from pr, the kernel workflow
has to be run first from a push action.

This will permit as a side effect to test merged commits and catch commit
that may cause regression in kernel compilation even outside the github
system.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

.github/workflows/kernel.yml

@@ -7,6 +7,12 @@ on:
       - 'include/kernel-*'
       - 'package/kernel/**'
       - 'target/linux/generic/**'
+  push:
+    paths:
+      - '.github/workflows/kernel.yml'
+      - 'include/kernel-*'
+      - 'package/kernel/**'
+      - 'target/linux/generic/**'
 
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   contents: read