Allow overriding Rust toolchain in tox runs (#12827)

It can be convenient to use different versions of the Rust toolchain for
various `tox` runs.  This can already be done externally to `tox`
by a stateful `rustup override set <x>` followed by clearing the
override after the tox run, but allowing the environment variable to
pass through lets it be localised to just a single run.

tox.ini

@@ -16,6 +16,7 @@ setenv =
   QISKIT_TEST_CAPTURE_STREAMS=1
   QISKIT_PARALLEL=FALSE
 passenv = 
+  RUSTUP_TOOLCHAIN
   RAYON_NUM_THREADS
   OMP_NUM_THREADS
   QISKIT_PARALLEL