Fix indefinite block when Config struct is initialized separately

[egibs]

If we use something like:

_, err := action.Scan(ctx, malcontent.Config{
	IncludeDataFiles: true,
	ScanPaths:        []string{testFile},
	RuleFS:           []fs.FS{rules.FS, thirdparty.FS},
})

if err != nil {
	t.Fatalf("Scan(%q) error = %v, want nil", testFile, err)
}

The new channel-based scanner pool will block indefinitely because c.Concurrency will default to 0 so no scanners will be created. This ensures that we initialize the pool regardless of how the Config is initialized.