feat(github): Allow to use custom runner from prj config

By default public runner will be used it can be overload
with custom value, in:

project settings / actions / secrets / RUNNER=private..

Origin: SiliconLabsSoftware/z-wave-engine-application-layer#73
Relate-to: #100
Forwarded: #134
Signed-off-by: Philippe Coval <philippe.coval@silabs.com>

Author: rzr

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
       contents: read
     env:
       project-name: z-wave-protocol-controller  # Align to docker (lowercase)
-    runs-on: ubuntu-22.04
+    runs-on: ${{ vars.RUNNER || 'ubuntu-24.04' }}
     steps:
       # yamllint disable-line rule:line-length
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2