Harden Linux manager executable lookup to prevent PATH hijacking

[mmorrissette-devolutions]

Motivation

• Close a local privilege-escalation vector where apt, dnf, and pacman binaries were discovered from the user PATH and later invoked under an elevator (sudo/pkexec), which allowed a PATH-hijacked binary to be run as root after user approval.
• Restrict manager executable discovery to known system locations to retain required elevation behavior while removing attacker-controlled PATH influence.

Description

• Changed FindCandidateExecutableFiles() in src/UniGetUI.PackageEngine.Managers.Apt/Apt.cs to only consider /usr/bin/apt and /bin/apt instead of using CoreTools.WhichMultiple("apt").
• Changed FindCandidateExecutableFiles() in src/UniGetUI.PackageEngine.Managers.Dnf/Dnf.cs to only consider /usr/bin/dnf5, /usr/bin/dnf, /bin/dnf5, and /bin/dnf and removed WhichMultiple usage for dnf candidates.
• Changed FindCandidateExecutableFiles() in src/UniGetUI.PackageEngine.Managers.Pacman/Pacman.cs to only consider /usr/bin/pacman and /bin/pacman instead of using CoreTools.WhichMultiple("pacman").
• Preserved the rest of the managers' behavior and the existing elevation flow, so the first available trusted path is still selected for operations that require admin rights.

Testing

• No automated unit or integration tests were executed because the environment lacks the .NET toolchain; dotnet --version returned command not found so dotnet test could not be run.
• Performed local code inspection and diff verification to ensure only the candidate discovery lists were modified and no operation/elevation code paths were changed.

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Codex Task