Always concatenate the default filters with the user-provided filters.
This ensures that when `paths-ignore` is provided,
we begin with the default path inclusions,
not all YAML files.
This makes the `paths-ignore-only` integration test variant
under `filters` pass.
The handling of `paths` is unchanged:
if provided, this overrides the default filters.
Quote the paths to the CodeQL CLI dist or autobuild scripts when invoked.
This unblocks integration testing, since our integration tests always use
a CLI with a space in its directory name.
