Fix Rust compiler build error (Kubernetes dependency)

[guyjacob]

The Python Kubernetes client depends on the cryptography package, which apparently requires the Rust compiler, and it currently fails building with the message below (see also here).

Trying the suggested step 1 first.

    running build_rust

        =============================DEBUG ASSISTANCE=============================
        If you are seeing a compilation error please try the following steps to
        successfully install cryptography:
        1) Upgrade to the latest pip and try again. This will fix errors for most
           users. See: https://pip.pypa.io/en/stable/installing/#upgrading-pip
        2) Read https://cryptography.io/en/latest/installation.html for specific
           instructions for your platform.
        3) Check our frequently asked questions for more information:
           https://cryptography.io/en/latest/faq.html
        4) Ensure you have a recent Rust toolchain installed.
        =============================DEBUG ASSISTANCE=============================

    error: Can not find Rust compiler

[guyjacob]

It already passed the stage where it was failing ("Prepare aws cli" stage in build_base). So looks like a valid fix.