Remove the extra jlpm run build in the workflow

[jtpio]

An extra jlpm run build was added to the CI workflow in #353:

jupyter-collaboration/.github/workflows/test.yml

Line 74 in c52fa20

jlpm build

Although it was not clear why this was needed, as discussed here: #353 (review)

It would be great to check if it could be removed. Maybe it's hiding another issue of how the repo is currently set up.

cc @davidbrochart @krassowski