Environment variables referenced in `activation.env` aren't resolved using deno shell

[Wumpf]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

[activation.env]
CARGO_TARGET_DIR = "$PIXI_PROJECT_ROOT/target_pixi"

Issue description

I tried using unix style env vars within activation.env like so:

[activation.env]
CARGO_TARGET_DIR = "$PIXI_PROJECT_ROOT/target_pixi"

This worked fine on Linux and Mac, but on Windows it didn't resolve the env var, thus in this case using literally "$PIXI_PROJECT_ROOT/target_pixi" as the target dir.

As a workaround I had to use

[target.win-64.activation.env]
CARGO_TARGET_DIR = "%PIXI_PROJECT_ROOT%/target_pixi"

See rerun-io/rerun@0471b2d

Expected behavior

Resolve env vars the same on all platforms

[Hofer-Julian]

At the moment, we add these to the activation script, so cmd will be used on Windows.

I agree that it would be a nice improvement to let that be handled by deno task shell instead. Let's evaluate first how many existing manifests we would break that way.