Inline environment activation

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

Similar to what @agriyakhetarpal has done for `conda run` in https://github.com/conda/conda/pull/15534, we can save a few subprocesses by inlining the activation code in the script. This is what Pixi's implementation does too. But sadly that means that the activation will won't be updated automatically if we install more packages with activation scripts. So we would need to force regenerate the shortcuts via a post-transaction plugin or something, if we do see that the environment now has additional activation scripts.

### Why is this needed?

Saves some startup overhead!

### What should happen?

Faster startup without compromising features or robustness.

### Additional Context

_No response_

Start by reading the linked conda PR #15534 and comparing Pixi's implementation, then trace menuinst's environment activation and shortcut generation entry points. The issue does not name files or tests; done means faster startup while preserving existing activation behavior and handling newly installed activation scripts through regeneration.

Written by the indexing model from the issue text.