Multi-platform environment installation

[trchen1033]

Hello Pixi Devs,

I would like to be able to install multiple platform variants of the same environment name in a workspace. For example, both the linux-64 variant and linux-aarch64 variant of the default environment to coexist.

The initial motivation was to be able to prebuild environments for some offline IoT devices from beefy build servers. Later as I started prototype the idea I realized that it is likely the right thing to do even in the general use case.

e.g. consider a shared NFS home mount, the "native" (a.k.a. environment.best_platform()) might not be consistent in the same workspace when worked from different workstation. IMO an environment installation should always associate with both the environment name and the targeted platform.

Here I already have a quick prototype that sort of works: 5fa7c1b

It adds --platform argument to pixi install and pixi shell. I've tested pixi shell --platform linux-aarch64 on a Debian multiarch host to launch an aarch64 environment.

Please let me know what's your thoughts on the general direction of the change, and how should we implement this properly and get it reviewed. Thanks for your time!

Here's a copy of the commit message:

[WIP] Adding multi-platform installation support

This patch decouples environment installation (./.pixi/envs/${env}) from
the current native platform.

The existing behavior was that when an environment gets installed, it picks
the platform variant by matching the native platform against the supported
platforms declared by the workspace.

This approach has a couple drawbacks:

• When a workspace is NFS mounted by multiple workstation, it is wrong to
  assume all workstation runs the same platform. For example, we have a
  namespace conflict here when both an aarch64 linux and a x86_64 linux
  attempt to install the default environment, as they will both install into
  ./.pixi/envs/default.

• Some Linux distros support installing shared libraries from multiple archs,
  and run foreign binaries seamlessly through qemu-user. Notably Debian
  multiarch. It is useful to be able to run native pixi but install environments
  for an emulated arch.

• It is also useful to be able to install environments on a build server which
  will be tarred and transferred to a target environment that doesn't have
  internet access. e.g. IoT platforms, industrial computers, e.t.c.

This patch does the following things:

• Environment installation path changed to ./.pixi/envs/${env}/{$platform}.

• Adds PIXI_ENVIRONMENT_PLATFORM envvar to indicate the targetting platform of
  the current environment.
  By the way there is the similarly named PIXI_ENVIRONMENT_PLATFORMS variable
  for the list of supported platforms. Not sure how is that useful at all.

• Adds --platform argument to pixi install to select the set of platform
  variants to install.

• Adds --platform argument to pixi shell to select alternative platform
  variant to activate.