`qmk flash` disappeared from the `qmk` command itself.
See `qmk_cli/script_qmk.py`:
```
This program can be run from anywhere, with or without a qmk_firmware
repository. If a qmk_firmware repository can be located we will use that
to augment our available subcommands.
```
By pointing QMK_HOME to our firmware sources, we can make it re-appear.
Change-Id: I5348fff546e864773063fb1f712bf2abd198972e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11627
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
