Add CLI function to snapshot the environment

[OleBialas]

Added nmaci snapshot-environment which print a snapshot of the current environment.
This can be used in CI to test that the executed notebooks are not accidentally installing their own dependencies.

Intended usage:

        - name: Snapshot Python environment
          run: |
            nmaci snapshot-environment \
              > "$RUNNER_TEMP/environment-before.txt"

        - name: Process notebook
          # Existing notebook execution step
          run: |
            nmaci process-notebooks "${{ matrix.notebook }}" --execute

        - name: Verify Python environment was not modified
          if: always()
          run: |
            nmaci snapshot-environment \
              > "$RUNNER_TEMP/environment-after.txt"

            if ! diff -u \
              "$RUNNER_TEMP/environment-before.txt" \
              "$RUNNER_TEMP/environment-after.txt"
            then
              echo "::error title=Environment modified::${{ matrix.notebook }} changed installed Python packages"
              exit 1
            fi