build: Update snapcraft

snap/snapcraft.yaml

@@ -65,14 +65,10 @@ parts:
     plugin: flutter
     source: .
     override-build: |
-      # Switch to correct Flutter version
-      source .github/workflows/versions.env
-      flutter_path=$(which flutter)
-      flutter_base=$(dirname $(dirname $flutter_path))
-      git -C $flutter_base checkout $FLUTTER_VERSION
-      flutter doctor
       # Workaround for Flutter build error:
       rm -rf build
+      flutter pub get
+      flutter gen-l10n
       craftctl default
     build-packages:
       - libsecret-1-dev