fix: make sure the config is writable after being copied

[SuperSandro2000]

Description

When the config file is copied on NixOS from the nix store the file is readonly for everyone and sunshine fails to save changes in the web ui.

Screenshot

Issues Fixed or Closed

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Bundle Report

Bundle size has no change ✅

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 11.34%. Comparing base (458a4a5) to head (60cb836).

Files with missing lines	Patch %	Lines
src/config.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4249      +/-   ##
==========================================
- Coverage   11.34%   11.34%   -0.01%     
==========================================
  Files          92       92              
  Lines       17623    17625       +2     
  Branches     8109     8110       +1     
==========================================
  Hits         2000     2000              
- Misses      14759    14919     +160     
+ Partials      864      706     -158     

Flag	Coverage Δ
Linux-AppImage	11.55% <0.00%> (-0.01%)	⬇️
Windows-AMD64	12.41% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/config.cpp	0.00% <0.00%> (ø)

... and 22 files with indirect coverage changes

[ReenigneArcher]

To confirm, the changes in the PR affect the apps.json, not the sunshine.conf. You are having issues saving changes to apps or to the configuration?