feat(core): pass parameters as env vars to scripts (and renku.api)

[Panaetius]

closes #2563

Parameters, Input, Outputs are now passed as RENKU_ENV_{name} environment variables. For parameters, this is picked up in renku.api.Parameter so a user can changes it's value.

Example script.py:

from renku.api import Parameter
print(Parameter("test", "hello world"))

Run like

$ renku run --name run1 -- python script.py > output
$ cat output 
hello world
$ renku workflow execute -p toil --set test=goodbye run1
$ cat output 
goodbye

Enabling renku.api.{Input,Output} to pick up modified values will be done in issue #2597

[vigsterkr]

🚢