Update TweakAtZ.py

[GregValiant]

Description

Add support for "One at a Time" print sequence.

Fixed a bug at line 747 by moving the statement to just below where the variable "change_retract_amt" is declared.

This fixes... OR This improves... -->

Type of change

• [ X] Bug fix (non-breaking change which fixes an issue)
• [ X] New feature (non-breaking change which adds functionality)

Test Configuration:

• Operating System: Win 10 Pro
  Cura 5.11, 5.10

Checklist:

• [ X] My code follows the style guidelines of this project as described in UltiMaker Meta and Cura QML best practices
• [ X] I have read the Contribution guide
• [ X] I have commented my code, particularly in hard-to-understand areas
• [ X] I have uploaded any files required to test this change

[github-actions]

Test Results

24 506 tests  ±0   24 504 ✅ ±0   49s ⏱️ -2s
     1 suites ±0        2 💤 ±0 
     1 files   ±0        0 ❌ ±0 

Results for commit 47077e6. ± Comparison against base commit 6e3776e.