This was always a terrible idea independently of whether it crashes.
Stop doing it!
This commit was verified by running nix-shell on a trivial derivation
with --debug --verbose to get the vomit-level output of the shell rc
file and then diffing it before/after this change. I have reasonable
confidence it did not regress anything, though this code is genuinely
really hard to follow (which is a second reason that I split it into two
fmt calls).
Fixes: #533
Change-Id: I8e11ddbece2b12749fda13efe0b587a71b00bfe5
