Add rust to binder postBuild

In Qiskit#7658 we updated added rust code to the Qiskit build environment.
This was done to accelerate performance critical portions of the
library. However, in Qiskit#7658 we overlooked the binder tests which are used
to perform image comparisons for visualizations in a controlled
environment. The base binder docker image does not have the rust
compiler installed, so we need to manually install it prior to running
pip to install terra. This commit takes care of this and adds rust to
the binder environment.

Author: mtreinish

postBuild

@@ -2,6 +2,11 @@
 # https://github.com/Qiskit/qiskit-terra/pull/4544/
 # See https://mybinder.readthedocs.io/en/latest/config_files.html?#postbuild-run-code-after-installing-the-environment
 
+# Install rust compiler to build compiled extensions
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs -o rustup.sh
+bash ./rustup.sh -y
+source $HOME/.cargo/env
+
 # Dependencies
 # - matplotlib: for MPL drawer
 # - pylatexenc: for MPL drawer