`$MIX_HOME` is used by mix(1) (for `~/.mix` equivalent). But the Hex
plugin does not use this variable to store `~/.hex`. Instead, it
apparently still relies on `$HOME` only.
So set and export both variables.
If I understand the code correctly, it should be enough to point mix(1)
to the packages we add to the source archive for offline build.
There should be no need to mess with `$HOME` or `$XDG_*_HOME`.
... by using the `override` directive to enforce our internal value.
As a reminder, this a hack to convince mix(1) to work offline and use a
copy of the Hex.pm cache provided in the source archive.
Note that this file was moved from rabbitmq-common. It has no use there
and it avoids the need to go through the entire CI pipeline if we want
to change this file.
The `stats.mk` file was also moved to the new `mk` subdirectory to avoid
file pollution at the root of this project.
Jean-Sébastien Pédron
Jean-Sébastien Pédron