borgbackup: cleanup inputs

tox/detox are removed; they are used to test multiple python
implementations in parallel, which isn't really appropriate for a Nix
install check.  Also, because these were added to the propagated build
inputs, lots of unnecessary stuff ended up in the resulting closure.
Note that removing tox does not imply tests are not run; they never were
to begin with; this is a separate issue.

This fixes https://github.com/NixOS/nixpkgs/issues/16171

Also, for "correctness" ordinary build-time dependencies are added to
`buildInputs`; only inputs that are to be added to the python path are
added to `propagatedBuildInputs`.