--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

Start by running the reported octocatalog-diff command with --output-file and multiple nodes (-n node1,node2). Trace the --output-file handling for each node and verify that the output is accumulated rather than replaced. Done means octodiff.output contains diffs for both node1 and node2.

Written by the indexing model from the issue text.