backup: fix working directory change

[endorama]

backup.Perform was using os.Chdir, which changes working
directory for the entire process. This in practice did not
have any effect, but is the potential source of future bugs
as it's an unexpected occurrence.

Solve this by restoring working directory after executing
the backup.
A dedicated test case is provided.

[codecov]

Codecov Report

Attention: Patch coverage is 74.07407% with 7 lines in your changes missing coverage. Please review.

Project coverage is 58.23%. Comparing base (b3a9eaf) to head (e961763).
Report is 51 commits behind head on main.

❗ Current head e961763 differs from pull request most recent head 82301b7

Please upload reports for the commit 82301b7 to get more accurate results.

Files	Patch %	Lines
internal/backup/perform.go	74.07%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   51.70%   58.23%   +6.53%     
==========================================
  Files          30       38       +8     
  Lines         441      510      +69     
==========================================
+ Hits          228      297      +69     
+ Misses        187      178       -9     
- Partials       26       35       +9     

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