ci: only test build quickgui if the flutter app was changed

[flexiondotorg]

Description

Test builds of the Quickgui Flutter app are always being triggered. This patch only triggers test builds of the app, if the app was actually changed.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas