ci: freeze macos runners at 13

Macos 13 stays on x86, macos 14 will be on arm64. Unfortunately arm64 mac runners don't support virtualization:
https://docs.github.com/en/actions/using-github-hosted-runners/about-larger-runners/about-larger-runners#limitations-for-macos-larger-runners

See: actions/runner-images#9255
And: actions/runner-images#9254

.github/workflows/tests-weekly.yaml

@@ -19,7 +19,8 @@ jobs:
       fail-fast: false
       matrix:
         python: ["3.11"]
-    runs-on: macos-latest
+        os: [macos-13]
+    runs-on: ${{ matrix.os }}
     timeout-minutes: 120
     steps:
       - name: Install Multipass
@@ -65,7 +66,8 @@ jobs:
       fail-fast: false
       matrix:
         python: ["3.11"]
-    runs-on: macos-latest
+        os: [macos-13]
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Install Multipass
         run: |

.github/workflows/tests.yaml

@@ -161,9 +161,11 @@ jobs:
           sg lxd -c ".tox/.tox/bin/tox run --skip-pkg-install --no-list-dependencies --colored yes -e integration-${{ matrix.python.tox-version }}"
   integration-smoketests-macos:
     strategy:
+      fail-fast: false
       matrix:
         python: ["3.11"]
-    runs-on: macos-latest
+        os: [macos-13]
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Install Multipass
         run: |