--output_file does not support multiple nodes

> Description of problem

- What did you do?
- Use the `--output-file` option for octocatalog-diff while running it against multiple target nodes
- What happened?
- The output file only contains the last analysed node
- What did you expect to happen?
- The output file contains the diffs of *all* analysed nodes
- How can someone reproduce the problem?
- Run octocatalog-diff with the `--output-file` option and use multiple nodes (`-n node1,node2`)

> Command used and debugging output

```
$ octocatalog-diff -f production -t target-branch --display-detail-add -n node1,node2 -o octodiff.output
I, [2022-06-21T10:55:51.744356 #123224] INFO -- : Catalogs compiled for node1
I, [2022-06-21T10:55:51.793838 #123224] INFO -- : Catalogs compiled for node2
I, [2022-06-21T10:55:52.171791 #123224] INFO -- : Diffs computed for node1
I, [2022-06-21T10:55:52.253632 #123224] INFO -- : Diffs computed for node2
I, [2022-06-21T10:55:52.353417 #123224] INFO -- : Wrote diff to octodiff.output
I, [2022-06-21T10:55:52.354721 #123224] INFO -- : Wrote diff to octodiff.output
$ head octodiff.output
diff production/node2 target-branch/node2
```

> Platform and version information

- Your OS: Ubuntu 18.03
- Your Ruby version: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]
- Your version of Puppet: 6.27.0
- Your version of octocatalog-diff: octocatalog-diff 2.1.0

Inizia eseguendo il comando octocatalog-diff indicato con --output-file e più nodi (-n node1,node2). Traccia la gestione di --output-file per ogni nodo e verifica che l'output venga accumulato anziché sostituito. Il lavoro è completato quando octodiff.output contiene i diff sia per node1 sia per node2.

Scritto dal modello di indicizzazione a partire dal testo della issue.