Don't use `spack env depfile`

[msimberg]

spack 1.0 will have parallel package builds by default. As far as I understand, this will also eventually use a build server, exactly as spack env depfile. Therefore depfile will be removed eventually, though not in 1.0. stackinator needs to be refactored to not use depfile sometime after spack 1.0 is released. The refactoring should be compatible with pretty much any spack version.

[bcumming]

I watched the presentation on this topic:
https://www.youtube.com/watch?v=HW0y5VNvuWk

It looks like this is still quite immature - e.g. the author was not aware that ninja can be used with a make server if you build a specific fork.

So I agree we could do away with the depfile feature, but we might have to wait until it is a robust solution (the ability to concurrently build independent packages is a big speed win)