passAsFile passes the values of Nix bindings to the builder as
files, so if those values contained references, they wouldn't end up
in the inputDerivation output. To fix that, append the contents of
every such passed file to the output.
We only have shell builtins in this derivation, so we can't use cat.
The only way I know of appending the contents of one file to another
using only shell builtins is as I've done here, but it requires
putting the contents of the file on echo's argv. This might end up
causing problems with large files. Regardless, I think we should try
this, as a failure is better than silently producing an incorrect
result like the previous behavior.
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