ci: Show flavour in about this app and sentry

[M123-dev]

What

Something we didn't need until know. But we will certainly need it in the future #3427

Now we show the app flavour in the "about this app" section as well as sending it to sentry.

Why a global variable?

The value won't change after the build and I put it in the main.dart because I didn't know where else to put. A single dedicated file seems again too much. If we later find anything to pair it with why not?

[teolemon]

LGTM 👌

[codecov-commenter]

Codecov Report

Merging #3429 (20fc559) into develop (cf3ce7f) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop    #3429      +/-   ##
===========================================
- Coverage    11.30%   11.30%   -0.01%     
===========================================
  Files          260      260              
  Lines        12555    12557       +2     
===========================================
  Hits          1419     1419              
- Misses       11136    11138       +2     

Impacted Files	Coverage Δ
...kages/smooth_app/lib/helpers/analytics_helper.dart	5.00% <0.00%> (-0.13%)	⬇️
packages/smooth_app/lib/main.dart	14.40% <0.00%> (-0.13%)	⬇️
...pp/lib/pages/preferences/user_preferences_faq.dart	7.50% <0.00%> (ø)

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