check-invariants: Simplify list comprehension

Rather than iterating over a list and check for membership in another
list, just iterate on the other list since we already know it is a
strict subset.

No behavioral changes.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

bin/packaging-check-invariants

@@ -176,11 +176,10 @@ class InvariantChecker:
         # for instance, for branch `debian/bullseye-security`, yield
         # `apertis/v2022` and `apertis/v2023dev2` as they are both based on Bullseye
         downstream_channels = set(
-            branch_split_addon(name)[0]
-            for name, channel in self.data.channels.items()
-            if channel.source.distribution == source.distribution
-            and channel.source.release == source.release
-            and name in package.git.branches.keys()
+            branch_split_addon(c)[0]
+            for c in channels
+            if self.data.channels[c].source.distribution == source.distribution
+            and self.data.channels[c].source.release == source.release
         )
         descendant_channels = set(
             branch_split_addon(b)[0] for b in branch.descendant_branches