feat: add template args for env vars in tasks as well

[tdejager]

Description

Fixes #5446

How Has This Been Tested?

I tested this with the following toml:

[workspace]
name = "template-env-test"
version = "0.1.0"
channels = ["conda-forge"]
platforms = ["osx-arm64"]

[tasks]
greet = { cmd = "echo $GREETING", env = { GREETING = "hello {{ name }}" }, args = [{ arg = "name", default = "world" }] }

json-env = { cmd = "echo $CONFIG", env = { CONFIG = '{"backend": "{{ backend }}"}' }, args = [{ arg = "backend", default = "numpy" }] }

[tasks.dep]
cmd = "echo BACKEND=$BACKEND"
env = { BACKEND = "{{ backend }}" }
args = [{ arg = "backend", default = "fallback" }]

[tasks.run-cupy]
depends-on = [{ task = "dep", args = ["CuPy"] }]

pixi run greet                     # hello world
pixi run greet pixi              # hello pixi
pixi run json-env               # {"backend": "numpy"}
pixi run json-env cupy      # {"backend": "cupy"}
pixi run run-cupy              # BACKEND=CuPy

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude Code

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[nichmor]

why it wasn't needed before?

[tdejager]

Yeah it should've been I think. Now I just reckoned it would happen more quickly.

[baszalmstra]

Claude said:

Missing test coverage (high priority):: There are no tests for the non-happy path.

docs/workspace/advanced_tasks.md — MiniJinja Templating section (around line 335): Currently says "Task commands defined in the manifest support MiniJinja templating syntax." This is now factually incomplete — template strings also work in env values. This sentence and the section's examples need updating.

docs/workspace/advanced_tasks.md — Environment Variables section (around line 464): Describes env behavior but makes no mention of template string support. At minimum, add a note and example showing {{ }} syntax in env values.

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The test coverage thing for non-happy path is fine I think.