snap: Add rust to build-packages

[freyes]

When building the snap on s390x some Python modules need to compile their extensions (rust based), this is not needed on amd64 since pypi has compiled packages available.

This change add rustc, cargo and pkg-config to the list of build-packages in the snapcraft.yaml definition.

Description of change

Checklist

• Are all your commits [logically] grouped and squashed appropriately?

[freyes]

build log on s390x - https://launchpadlibrarian.net/758496706/buildlog_snap_ubuntu_bionic_s390x_charm-tools.2x_BUILDING.txt.gz

...
  copying src/cryptography/hazmat/bindings/_rust/openssl/__init__.pyi -> build/lib.linux-s390x-3.6/cryptography/hazmat/bindings/_rust/openssl
  copying src/cryptography/hazmat/bindings/_rust/openssl/x25519.pyi -> build/lib.linux-s390x-3.6/cryptography/hazmat/bindings/_rust/openssl
  running build_ext
  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/ for specific
         instructions for your platform.
      3) Check our frequently asked questions for more information:
         https://cryptography.io/en/latest/faq/
      4) Ensure you have a recent Rust toolchain installed:
         https://cryptography.io/en/latest/installation/#rust
  
      Python: 3.6.9
      platform: Linux-5.4.0-200-generic-s390x-with-Ubuntu-18.04-bionic
      pip: n/a
      setuptools: 59.6.0
      setuptools_rust: 1.1.2
      rustc: n/a
      =============================DEBUG ASSISTANCE=============================
  
  error: can't find Rust compiler
  
  If you are using an outdated pip version, it is possible a prebuilt wheel is available for this package but pip is not able to install from it. Installing from the wheel would avoid the need for a Rust compiler.
  
  To update pip, run:
  
      pip install --upgrade pip
  
  and then retry package installation.
  
  If you did intend to build this package from source, try installing a Rust compiler from your system package manager and ensure it is on the PATH during installation. Alternatively, rustup (available at https://rustup.rs) is the recommended way to download and update the Rust compiler toolchain.
  
  This package requires Rust >=1.48.0.
  ----------------------------------------
  ERROR: Failed building wheel for cryptography