[bug#66801] [PATCH] build-system: Add mix-build-system.

[Pierre-Henry Fröhring]

      For the record, I get these hashes on current master:

      grafting '/gnu/store/dapdrihxaxfxfcky51sr4607a79msgp6-elixir-hex-2.0.5'
            -> '/gnu/store/blg85m7a4jd9cdyc6q9m96lnxf9d97h0-elixir-hex-2.0.5'

I do not know exactly how to hash is computed.  The previous build of 
elixir-hex has
been done after rebasing the commits on top of the commit `06f25a9a8' of 
master.  For
the record:
┌────
│ git show --oneline -s
└────

┌────
│ 0fcd231da (HEAD -> master) gnu: Add elixir-machete.
└────

┌────
│ ./pre-inst-env guix build --check elixir-hex
└────

┌────
│ The following graft will be made:
│    /gnu/store/1732zx2s5afv1da5vhik0gr923frvg35-elixir-hex-2.0.5.drv
│ applying 1 graft for elixir-hex-2.0.5 ...
│ grafting '/gnu/store/dapdrihxaxfxfcky51sr4607a79msgp6-elixir-hex-2.0.5' -> 
'/gnu/store/hak603f9adwad1vrlqkqn6ywgissfb13-elixir-hex-2.0.5'...
│ successfully built 
/gnu/store/1732zx2s5afv1da5vhik0gr923frvg35-elixir-hex-2.0.5.drv
│ successfully built 
/gnu/store/1732zx2s5afv1da5vhik0gr923frvg35-elixir-hex-2.0.5.drv
│ /gnu/store/hak603f9adwad1vrlqkqn6ywgissfb13-elixir-hex-2.0.5
└────

      And you still forgot the reroll-count.  Don't forget the reroll-count.
Well, what I assumed the "reroll-count" was was wrong.  You mean the `v<n>' in 
`[PATCH
v<n>]' in the subject of the patches? In my next email, I will send the same 
patches
except that I will use the `--reroll-count=3' option to build them.