Issue #1049 Restrict PATH to pixi and Modflow6

[JoerivanEngelen]

Fixes #1049

Description

This PR restricts the PATH environmental variable for critical jobs on the TeamCity agent.

Checklist

• [x] Links to correct issue
• [ ] Update changelog, if changes affect users
• [x] PR title starts with Issue #nr, e.g. Issue #737
• [ ] Unit tests were added
• [ ] If feature added: Added/extended example

[Manangka]

Nice! Good to have full control over our dependencies again.