[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]>

composer.json

@@ -31,6 +31,7 @@
         "phpdocumentor/type-resolver": "^1.1.0"
     },
     "require-dev": {
+        "ext-pdo_sqlite": "*",
         "friendsofphp/php-cs-fixer": "^2",
         "illuminate/config": "^6 || ^7 || ^8",
         "illuminate/view": "^6 || ^7 || ^8",