ci: fix signed-off-by checks in master

Only check signed commits for PRs.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>
diff --git a/.github/workflows/mynewt.yaml b/.github/workflows/mynewt.yaml
index 2cc4d79..b4f5e07 100644
--- a/.github/workflows/mynewt.yaml
+++ b/.github/workflows/mynewt.yaml
@@ -21,6 +21,7 @@
         free
         pwd
     - name: Signed commit check
+      if: ${{ github.event_name == 'pull_request' }}
       run: |
         ./ci/check-signed-off-by.sh
     - name: Mynewt install
diff --git a/.github/workflows/sim.yaml b/.github/workflows/sim.yaml
index 39d09ca..8623568 100644
--- a/.github/workflows/sim.yaml
+++ b/.github/workflows/sim.yaml
@@ -41,6 +41,7 @@
         free
         pwd
     - name: Signed commit check
+      if: ${{ github.event_name == 'pull_request' }}
       run: |
         ./ci/check-signed-off-by.sh
     - name: Install stable Rust