fix: channel priority logic

[ruben-arts]

The issue described in #1331 can be simplified into the added test:

[project]
name = "holoviews"
channels = ["a", "b"]
platforms = ["linux-64"]

[environments]
foo = ["foo"]

[feature.foo]
channels = ["a", "c", "b"]

The channels for env foo should be ["a", "c", "b"] but the current logic in v0.21.0 changes it to have the default feature channels always at the end resulting in ["c", "a', "b"].

We changed this back to keep the order of the features, and only add the missing channels from default last in the list.

So this fixes #1331