CICD: Don't generate build number for pull requests.

Prevent attempts to generate a build number for pull request builds as
they will fail due to the lack of permissions on the
`subsurface/nightly-builds` repository.

Signed-off-by: Michael Keller <github@ike.ch>

.github/workflows/android.yml

@@ -19,6 +19,7 @@ jobs:
 
     - name: atomically create or retrieve the build number
       id: build_nr
+      if: github.event_name == 'push'
       run: |
         cd .. # check out parallel to subsurface sources
         url="https://subsurface:${{ secrets.NIGHTLY_BUILDS }}@github.com/subsurface/nightly-builds"

.github/workflows/mac.yml

@@ -15,6 +15,7 @@ jobs:
       uses: actions/checkout@v1
     - name: atomically create or retrieve the build number
       id: build_nr
+      if: github.event_name == 'push'
       run: |
         cd .. # check out parallel to subsurface sources
         url="https://subsurface:${{ secrets.NIGHTLY_BUILDS }}@github.com/subsurface/nightly-builds"

.github/workflows/windows.yml

@@ -19,6 +19,7 @@ jobs:
 
     - name: atomically create or retrieve the build number
       id: build_nr
+      if: github.event_name == 'push'
       run: |
         cd .. # check out parallel to subsurface sources
         url="https://subsurface:${{ secrets.NIGHTLY_BUILDS }}@github.com/subsurface/nightly-builds"