feat: support PIXI_NO_INSTALL environment variable

[Shardz4]

Closes #5837.

Description

This PR adds support for the PIXI_NO_INSTALL environment variable to mirror the existing --no-install CLI flag.

Previously, pixi supported setting lockfile configurations through environment variables for --frozen (PIXI_FROZEN) and --locked (PIXI_LOCKED), but did not have an equivalent for --no-install. Users wanting a strictly read-only environment could not configure --no-install via .envrc alongside the other variables.

This change fixes that by tying the PIXI_NO_INSTALL env var directly to the clap argument parsing for all commands utilizing no_install.

Changes Made:

• Added env = "PIXI_NO_INSTALL" to NoInstallConfig in crates/pixi_cli/src/cli_config.rs.
• Added the same env var attribute to the standalone no_install argument in update.rs (Args) and workspace/platform.rs (AddArgs and RemoveArgs).
• Updated the lockfile documentation in docs/workspace/lockfile.md to document the new PIXI_NO_INSTALL variable.

Fixes #5837

How Has This Been Tested?

• Confirmed all commands with a --no-install flag correctly display [env: PIXI_NO_INSTALL=] in their --help output (e.g., pixi add --help, pixi run --help, pixi workspace platform add --help).
• End-to-end functionality verification: Tested PIXI_NO_INSTALL=true pixi add <pkg> and confirmed that while the manifest and lockfile are updated, the environment creation/installation step in .pixi/envs/default is successfully bypassed.
• Tested the control case (without the env var) and confirmed the environment installs correctly.
• Ran cargo check -p pixi_cli and cargo test -p pixi_cli to ensure there are no regressions or syntax errors.

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: Google Deepmind Antigravity Agent/ Claude Opus 4.6(thinking)

Prompt:

Add env var PIXI_NOINSTALL
Issue #5837 — Add env var PIXI_NOINSTALL
Problem Summary
Pixi has three related CLI flags that control lock-file and installation behavior.
The issue requests a PIXI_NOINSTALL (or similar) env var so that --no-install behavior can be set via environment variables — just like --frozen and --locked already can.