cmd: implement manifest command

This commit implements the `manifest` command for `image-builder`.
It will generate an osbuild manifest based on the given inputs,
e.g.:
```
$ ./image-builder manifest centos-9 qcow2
{"version":"2","pipelines":[{"name":"build","runner":",...
```

Note that there is an integration test but because of the depsolve
it will be slow. It will be skipped when doing `go test -short`.

cmd/image-builder/export_test.go

@@ -10,6 +10,7 @@ import (
 
 var (
 	GetOneImage = getOneImage
+	Run         = run
 )
 
 func MockOsArgs(new []string) (restore func()) {
@@ -48,7 +49,3 @@ func MockNewRepoRegistry(f func() (*reporegistry.RepoRegistry, error)) (restore
 		newRepoRegistry = saved
 	}
 }
-
-var (
-	Run = run
-)