chore: make skopeo tests optional on non linux/mac

.github/workflows/tests.yaml

@@ -63,9 +63,13 @@ jobs:
         if: ${{ startsWith(matrix.os, 'ubuntu') }}
         run: |
           sudo apt update
-          sudo apt install -y python3-pip python3-setuptools python3-wheel python3-venv libapt-pkg-dev skopeo
+          sudo apt install -y python3-pip python3-setuptools python3-wheel python3-venv libapt-pkg-dev
           export $(cat /etc/os-release | grep VERSION_CODENAME)
           pip install -U -r "requirements-${VERSION_CODENAME}.txt"
+      - name: Install external dependencies with homebrew
+        if: ${{ runner.os == 'Linux' || runner.os == 'macOS' }}
+        run: |
+          brew install skopeo
       - name: Configure environment
         run: |
           python -m pip install tox

snap/snapcraft.yaml

@@ -155,7 +155,7 @@ parts:
   skopeo:  # Copied from Rockcraft
     plugin: nil
     source: https://github.com/containers/skopeo.git
-    source-tag: v1.14.2
+    source-tag: v1.15.1
     override-build: |
       CGO=1 go build -ldflags -linkmode=external ./cmd/skopeo
       mkdir "$CRAFT_PART_INSTALL"/bin

tests/unit/utils/test_skopeo.py

@@ -16,12 +16,21 @@
 """Unit tests for skopeo wrapper."""
 
 import pathlib
+import platform
 from unittest import mock
 
 import pytest
 
 from charmcraft.utils.skopeo import Skopeo
 
+pytestmark = [
+    pytest.mark.xfail(
+        platform.system().lower() not in ("linux", "darwin"),
+        reason="Don't necessarily have skopeo on non Linux/mac platforms.",
+        strict=False,  # Allow them to pass anyway.
+    ),
+]
+
 IMAGE_PATHS = [  # See: https://github.com/containers/skopeo/blob/main/docs/skopeo.1.md#image-names
     "containers-storage:my/local:image",
     "dir:/tmp/some-dir",