## Changes
While they are a slice, we can identify a job cluster by its job cluster
key. A job definition with multiple job clusters with the same key is
always invalid. We can therefore merge definitions with the same key
into one. This is compatible with how environment overrides are applied;
merging a slice means appending to it. The override will end up in the
job cluster slice of the original, which gives us a deterministic way to
merge them.
Since the alternative is an invalid configuration, this doesn't change
behavior.
## Tests
New test coverage.
Pieter Noordhuis