Raising build errors if cargo is required and not found

[maliasadi]

The runtime requires an installation of cargo to build qir-stdlib from source (on macOS or by developer requires via the BUILD_QIR_STDLIB_FROM_SRC=ON CMake flag). Currently, if the developer forget to install cargo or the llvm rustup component, the build will be failed with the following error message:

catalyst/runtime/include/RuntimeCAPI.h:21:10: fatal error: 'qir_stdlib.h' file not found
#include "qir_stdlib.h"

This PR updates the build system of the runtime to throw errors at the proper build step with a clear message:

-- Building qir-stdlib from source.
CMake Error at CMakeLists.txt:74 (message):
  Missing cargo installation

  Please install "Rust" and the "llvm-tools-preview" rustup component for
  building qir-stdlib from source.  See the installation guideline for
  details:
  https://docs.pennylane.ai/projects/catalyst/en/stable/dev/installation.html

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/changelog.md on your branch with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[dime10]

👍