ci/parse: only show first error

There is no point in running the much slower `parse-each` part for each
interpreter/version. The CI job is not meant as a development tool that
should report all parse errors at once, but as a confirmation that no
parse errors are present on *different interpreter versions*.

Once this test fails, Eval, nixpkgs-vet and treefmt will most likely
fail as well - with more information for multiple parse errors.

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