repos: make clearer why newRepoRegistry is a var

This commit reworks the `newRepoRegistry` func so that its easier
to see that it is a variable so that it can be overriden by the
tests. In the tests we want to use the `testrepos` we get from
images and in the real implementation we want to use the full
repo loader with search-paths and extra repos.

cmd/image-builder/repos.go

@@ -53,7 +53,7 @@ func parseExtraRepo(extraRepo string) ([]rpmmd.RepoConfig, error) {
 	}, nil
 }
 
-var newRepoRegistry = func(dataDir string, extraRepos []string) (*reporegistry.RepoRegistry, error) {
+func newRepoRegistryImpl(dataDir string, extraRepos []string) (*reporegistry.RepoRegistry, error) {
 	var dataDirs []string
 	if dataDir != "" {
 		dataDirs = []string{dataDir}
@@ -87,3 +87,6 @@ var newRepoRegistry = func(dataDir string, extraRepos []string) (*reporegistry.R
 
 	return reporegistry.NewFromDistrosRepoConfigs(conf), nil
 }
+
+// this is a variable to make it overridable in tests
+var newRepoRegistry = newRepoRegistryImpl