nixpkgs: chez scheme update
nixpkgs: chez scheme update

Remove the parallel build[1], and update to the latest commit which
updates the .boot files and fixes a few bugs, too.

[1] I figured many builds on my dual-socket 12core would expose
problems, but I have a suspicion of that being an issue.

Signed-off-by: Austin Seipp <48c905ac3851f382b914247d97dd93273d5ff555@pobox.com>

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