Tor nightly builds are broken on Linux with Rust enabled (since 10/31)

Landing 74c1e44746a7d9c810f095177af88b7dbaafd3e3 in tor breaks our nightly builds, presumably because our Rust compiler (1.26.1) is too old for that:

   Compiling tor_allocate v0.0.1 (file:///var/tmp/build/tor-aa1ae1343a2e/src/rust/tor_allocate)
   Compiling smartlist v0.0.1 (file:///var/tmp/build/tor-aa1ae1343a2e/src/rust/smartlist)
  CC     src/lib/crypt_ops/src_lib_libtor_crypt_ops_a-crypto_curve25519.o
error[E0432]: unresolved import `std::alloc::System`
  --> tor_allocate/lib.rs:14:5

I think we have three options to fix that:

1. We build the nightlies with the Rust version tor needs.
2. We build only tor in the nightlies with the Rust version tor needs.
3. We disable compiling tor with Rust on nightlies

I think I prefer 2) here.