Fix visudo bugs; update the CI

[squell]

CC #833

This also makes some changes which should be fix the CI for at least Rust 1.80 by removing some dead code and other cobwebs. Every change has been put in a seperate commit for easy review.

The penultimate commit ("fix indentation") consists of whitespace-only changes.

In more detail, this PR functionally ensures that:

• Truncates the sudoers file correctly after it has shrunk
• Prints file source and position information of errors instead of just saying "syntax error".

Thereby Closing #833

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 57.48%. Comparing base (4f93acb) to head (865a7d3).
Report is 12 commits behind head on main.

❗ Current head 865a7d3 differs from pull request most recent head 13e7dcc

Please upload reports for the commit 13e7dcc to get more accurate results.

Files	Patch %	Lines
src/visudo/mod.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #843      +/-   ##
==========================================
- Coverage   57.98%   57.48%   -0.50%     
==========================================
  Files          75       75              
  Lines       10560    10421     -139     
==========================================
- Hits         6123     5991     -132     
+ Misses       4437     4430       -7     

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