fix(postins): allow running on rpm-ostree environments

[ykshek]

Description

Allow running postinst on rpm-ostree environments, more specifically for brew installs to allow Kernel Mode Setting.
Output of running the modified postinst script on Bazzite:

bazzite@bazzite:/var/home/username$ sudo /home/bazzite/postinst 
Loading uhid kernel module for DS5 emulation.
Setting CAP_SYS_ADMIN, CAP_SYS_NICE capabilities on Sunshine binary.
/usr/sbin/setcap cap_sys_admin,cap_sys_nice+p /var/home/linuxbrew/.linuxbrew/Cellar/sunshine-beta/2026.407.170539/bin/sunshine-2026.407.170539
CAP_SYS_ADMIN, CAP_SYS_NICE capabilities set on Sunshine binary.
Reloading udev rules.
Udev rules reloaded successfully.

And after running and restarting the sunshine service, the kernel mode setting capture method can be used.

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

[ykshek]

This change would be rather useful for ublue images as currently they also just runs the same commands to enable kernel mode setting capture, just manually or via the ujust helper script. Would be especially useful after a brew update, because the path of the executable changes every update and setcap needs to be run every time that happens to make sure capture doesn't break.