Tweaks to prefix-log (#18274)

- Explicitly use `mawk` instead of `awk`, since an extension of the
former is used
- Use `fflush` to reduce interleaving the output of different processes
& streams
- Move the `mawk` command to a shell function, instead of writing it
twice
- Look up the `SUPERVISOR_PROCESS_NAME` environment variable in `mawk`,
instead of reading it in the shell & using complex quoting to pass it to
`mawk`

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

changelog.d/18274.docker

@@ -0,0 +1 @@
+Make some improvements to the prefix-log script in the workers image.

docker/prefix-log

@@ -10,6 +10,9 @@
 # '-W interactive' is a `mawk` extension which disables buffering on stdout and sets line-buffered reads on
 # stdin. The effect is that the output is flushed after each line, rather than being batched, which helps reduce
 # confusion due to to interleaving of the different processes.
-exec 1> >(awk -W interactive '{print "'"${SUPERVISOR_PROCESS_NAME}"' | "$0 }' >&1)
-exec 2> >(awk -W interactive '{print "'"${SUPERVISOR_PROCESS_NAME}"' | "$0 }' >&2)
+prefixer() {
+    mawk -W interactive '{printf("%s | %s\n", ENVIRON["SUPERVISOR_PROCESS_NAME"], $0); fflush() }'
+}
+exec 1> >(prefixer)
+exec 2> >(prefixer >&2)
 exec "$@"