rerun and update shouldn't overwrite the plan index

[Panaetius]

Currently, if a rerun or update is executed, we add the created Activity to the ActivityGateway and that also adds its plan to the PlanGateway, overwriting the plan index if the plan was edited in the meantime.

Namely here, if the executed activity is linked to an old plan, it'll set the plans-by-name to the older version of the plan.

Full example to reproduce:

$ renku init test-wf
$ cd test-wf
$ renku run --name readme wc README.md > readme.wc
$ renku workflow edit -s input-1=Dockerfile readme
$ renku workflow show readme

Id: /plans/58f9ac8ac6ee45adab3132ab9f3fe02b
Name: readme
Command: wc Dockerfile > readme.wc
Success Codes:
Inputs:
        - input-1:
                Default Value: Dockerfile
                Position: 1
Outputs:
        - output-2:
                Default Value: readme.wc
                Position: 2

$ renku rerun readme.wc
$ renku workflow show readme

Id: /plans/add3ec0d64f84f5ca20d241e7c08fd11
Name: readme
Command: wc README.md > readme.wc
Success Codes:
Inputs:
        - input-1:
                Default Value: README.md
                Position: 1
Outputs:
        - output-2:
                Default Value: readme.wc
                Position: 2

this shows the old plan instead of the new one.