Set MIX_QUIET=1 when running mix format

With the current way of passing things to aphelia, sometimes
compilation output gets added to the top of the formatted file.

This is because `mix format -` will actually still use [`Mix.Shell.info/1`](https://hexdocs.pm/mix/1.17.3/Mix.Shell.html#c:info/1) to output thing like:

```
==> module
Compiling 56 files (.ex)
```

However `info` messages can be silenced when setting `MIX_QUIET`.
Docs: https://hexdocs.pm/mix/1.17.3/Mix.html#module-environment-variables

CHANGELOG.md

@@ -32,8 +32,11 @@ The format is based on [Keep a Changelog].
 * `mix format` is now passed the `--stdin-filename` argument which is
   required in some cases. The version of Mix is autodetected and this
   option is only passed when it is supported ([#319]).
+* `mix format` is now run with `MIX_QUIET` to supress compilation
+  output ([#326])
 * Beancount files are formatted without an error ([#309]).
 
+
 ## Internal
 * Improvements to formatter test framework, it is now possible to
   write tests that have additional data files ([#301]).

scripts/formatters/apheleia-mix-format

@@ -32,4 +32,4 @@ if [[ "${ver}" == "${higher_ver}" ]]; then
 fi
 args+=(-)
 
-exec "${args[@]}"
+MIX_QUIET=1 exec "${args[@]}"