* 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]>
Markus Podarandlaravel-ide-helper
Tom Witkowski