[GHA] Misc improvements (#1101)

* gha: be more specific which OS to use

Avoids "surprises" when Github changes the underlying version, like they
currently announce with a warning:
> Ubuntu-latest workflows will use Ubuntu-20.04 soon. For more details, see https://github.com/actions/virtual-environments/issues/1816

Note: basically right now "latest" is actually 18.04 and not 20.04

* composer fix-style

* gha: require league/flysystem:^1 for lumen

2.* doesn't work out of the box

* gha: explicit version of composer isn't necessary anymore

v2 is already the default

* Be explicit to require the pdo SQLite extension for tests

Co-authored-by: laravel-ide-helper <[email protected]>

.github/workflows/fix-code-style.yml

@@ -7,7 +7,7 @@ jobs:
   fix-style:
     name: Fix Code Style
     timeout-minutes: 15
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       COMPOSER_NO_INTERACTION: 1
 
@@ -20,7 +20,6 @@ jobs:
         with:
           php-version: 7.4
           coverage: none
-          tools: composer:v2
 
       - name: Install dependencies
         run: composer update --prefer-dist --no-progress