github: workflows: Replace macos-13 target with macos-15-intel

THe macos-13 runner is no longer available on github, replace it
with macos-15-intel to ensure builds on x86 mac are successful

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-22.04, macos-13, macos-14, windows-2022]
+        os: [ubuntu-22.04, macos-14, macos-15-intel, windows-2022]
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout