CI: Simplify Linux build

[stenzek]

Description of Changes

• Moves the package installation into the actions workflow. Don't need separate scripts now we're not doing Qt/WX builds anymore.
• Removes freetype/harfbuzz from the deps build, uses the system package instead. Avoids a potential conflict with fontconfig.
• Adds the GTK+ theme for Qt, improves native theme on GNOME (but you really shouldn't be using that broken DE...)
• Gets rid of additional bounds checks in the Flatpak, that the SDK was injecting.

Rationale behind Changes

Tidy up.

Suggested Testing Steps

Make sure AppImage and Flatpak still work.

[AmyRoxwell]

appimage still works!