haskell-modules/generic-builder.nix: use mktemp instead of TMPDIR

Using $TMPDIR here is problematic because it is not always cleared at
the end of each build, for instance when using "nix-shell --run
genericBuild". This can cause confusing errors when a nix-shell build
is trying to pull in dependencies from a previous build since it tries
to use older package conf files.

To fix, we can just use mktemp which will guarantee us a clean
directory for each build. Should have no effect in nix-build, but will
fix a common issue with using generic-builder in nix-shell.

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