move the nightly env into its own file

[keewis]

This avoids having to build binary dependencies on e.g. RTD in order to lock the dependencies.

This won't actually do anything besides accelerating the locking, until we change the upstream-dev ci. For that we need to have parcels-code/pixi-lock support the manifest-path or working-directory options of setup-pixi (cc @VeckoTheGecko), and we also need to change the commands we use to run the tests. I may be able look into both this week.

Ultimately we're working around two restrictions in pixi with this:

• conda dependencies are resolved first and cannot be changed when resolving pypi dependencies
• source dependencies are built into wheels when locking to figure out dependencies, which means this always happens first, regardless of whether the environment is then actually used.

For reference, I saw @TomNicholas did something very similar for virtualizarr a couple of months ago, except that is using pip to modify the environment.