Update pixi.toml to install Parcels from source

[VeckoTheGecko]

Fixes #2272

Before:

pixi run -e test-latest 'cd tests && python -c "import parcels"'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'parcels'

After:

pixi run -e test-latest 'cd tests && python -c "import parcels"'

This enables #2216 - how should we time that @erikvansebille to reduce disruption? After #2303 is merged?

[erikvansebille]

Looks good to me. Why should we wait until #2303? Is this change not completely perpendicular to that PR?

[VeckoTheGecko]

This change is, but #2216 isn't - I was wondering about that

[erikvansebille]

Maybe I don't understand what #2216 then is. Is it 'simply' a rename of the parcels directory to src?

In that case, let's indeed do it after #2303. That PR is ready, except for the failing assert on the UX hash, which fails in many more tests (right?)

[VeckoTheGecko]

Is it 'simply' a rename of the parcels directory to src?

Yes.

Let's do after #2303

[VeckoTheGecko]

For some reason generating the lock file is hanging (40m at least for a solve). Not sure why since this works locally. Going to see if I can create a minimal example and bring it to the prefix team