Fix Arch Linux dependency installation
The arch dependency script listed packages that are no longer available
in current Arch/CachyOS repositories:
- gstreamermm: removed from official repos (AUR only) and not referenced
anywhere in the OrcaSlicer build; the build uses plain gstreamer.
- webkit2gtk: replaced upstream by webkit2gtk-4.1.
Also switch the install command from `pacman -Syy` to `pacman -Syu` to
avoid the partial-upgrade pattern that Arch officially discourages on a
rolling-release distro.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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