Fail and retry on "Some index files failed to download"

In scripting apt operations, adding `--error-on=any` to `apt-get update`
is necessary to capture erros like "Some index files failed to
download". Otherwise, charms move onto package installations with a
stale index.

equivalent to: juju/charm-helpers#911
ref: https://bugs.launchpad.net/ubuntu/+source/apt/+bug/1693900

lib/charms/operator_libs_linux/v0/apt.py

@@ -837,7 +837,7 @@ def remove_package(
 
 def update() -> None:
     """Update the apt cache via `apt-get update`."""
-    subprocess.run(["apt-get", "update"], capture_output=True, check=True)
+    subprocess.run(["apt-get", "update", "--error-on=any"], capture_output=True, check=True)
 
 
 def import_key(key: str) -> str:

tests/unit/test_apt.py

@@ -511,7 +511,9 @@ def test_refreshes_apt_cache_if_not_found(self, mock_subprocess, mock_subprocess
             apt_cache_aisleriot,
         ]
         pkg = apt.add_package("aisleriot")
-        mock_subprocess.assert_any_call(["apt-get", "update"], capture_output=True, check=True)
+        mock_subprocess.assert_any_call(
+            ["apt-get", "update", "--error-on=any"], capture_output=True, check=True
+        )
         self.assertEqual(pkg.name, "aisleriot")
         self.assertEqual(pkg.present, True)
 
@@ -527,7 +529,9 @@ def test_raises_package_not_found_error(self, mock_subprocess, mock_subprocess_o
         ] * 2  # Double up for the retry after update
         with self.assertRaises(apt.PackageError) as ctx:
             apt.add_package("nothere")
-        mock_subprocess.assert_any_call(["apt-get", "update"], capture_output=True, check=True)
+        mock_subprocess.assert_any_call(
+            ["apt-get", "update", "--error-on=any"], capture_output=True, check=True
+        )
         self.assertEqual("<charms.operator_libs_linux.v0.apt.PackageError>", ctx.exception.name)
         self.assertIn("Failed to install packages: nothere", ctx.exception.message)