internal/ci: don't pin CUE v0.5 to import the github-workflow jsonschema

It seems like this repository, as well as many others like cuelang.org,
used the default in _cueCmd, as they did not supply the cue_cmd tag.

It also seems like we never noticed for over 18 months because
`cue import` and particularly our encoding/jsonschema package
have not changed significantly between v0.5 and now.

I noticed as I was trying to debug why cuelang.org's `go generate`
for internal/ci, which is copied from cue's, did not work consistently.
Now that Roger has implemented many improvements to jsonschema
for the v0.10 release, and particularly as we will fix even more bugs
such as the lack of proper support for jsonschema's oneOf,
it seems worthwhile to generate with master once again.

Other repositories which copy internal/ci all depend on cuelang.org/go
in go.mod, so the `go run` without a version should do the right thing.
For any of them that don't, like Unity, they can use the cue_cmd tag.

The main change is that we now handle required fields properly,
as well as some other minor changes thanks to Roger's fixes.
Note that the required fields change means we need to tweak cue.mod/usr.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ifd5796a779430c105b28f8d8a06f49457e5b0fcd
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199497
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>