Fix CI failures on Fedora and macOS platforms

[Copilot]

CI fails on macOS because Linux-only packages are in a global list that the macOS brew loop iterates over, and on Fedora because third-party apps requiring external repos (pinned to old Fedora versions) block the build.

Changes

• Platform-scoped package lists: Moved Linux-only packages (xclip, pip, bugwarrior, cronie, gnome-tweaks, dev libraries, etc.) into an [[ ${OS} == "Linux" ]] conditional block, matching the existing pattern used for coreutils on Darwin
• pip-based packages on macOS: Added pip3 install path for pynvim in the Darwin section instead of attempting brew install
• Optional packages (non-fatal): Separated brave-browser, google-chrome-stable, and insync into an optionalPackages list with its own install loop—failures are reported as warnings but don't exit non-zero

# Before: all packages in flat global list, macOS tries to brew install everything
packages+=(gnome-tweaks)
packages+=(libevdev-devel)
packages+=(brave-browser)

# After: platform-aware scoping
if [[ ${OS} == "Linux" ]]; then
    packages+=(gnome-tweaks)
    packages+=(libevdev-devel)
    optionalPackages+=(brave-browser)
fi

Also removed a duplicate xclip entry and fixed a typo in a comment.