CI: test binder startup

[bsipocz]

At another, much smaller traffic, project I run into the issue that the binder deployment stopped working without CI noticing.
Would be nice to work out a CI job that checks on that all the deployment options (binder, colab, or anything else we enable) works as expected (that case it was due to version incompatibilities),

[soma2000-lang]

@bsipocz working on this one.

[bsipocz]

I would advocate for dropping the env file in favour of reusing the requirements files for the deployments, after all we already test those and duplicating listing dependencies just creates more opportunities for failures.

[bsipocz]

(that being said, environment.yml is being tested in a job, so it apparently not adequate for picking up issues with the deployment, see #216 )