Also skip if missing 'gbm'

[MichaelChirico]

No description provided.

[jgabry]

Thank you!

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if a5c3f6c is merged into master:

• ✔️as_draws_array: 136ms -> 136ms [-0.71%, +0.65%]
• ✔️as_draws_df: 64.1ms -> 63.9ms [-1.49%, +0.89%]
• 🚀as_draws_list: 161ms -> 158ms [-1.95%, -0.99%]
• ✔️as_draws_matrix: 27.2ms -> 27.2ms [-0.4%, +0.82%]
• ✔️as_draws_rvars: 79.4ms -> 78.9ms [-1.25%, +0.11%]
• ✔️summarise_draws_100_variables: 697ms -> 698ms [-0.09%, +0.42%]
• ✔️summarise_draws_10_variables: 110ms -> 110ms [-0.79%, +0.37%]
  Further explanation regarding interpretation and methodology can be found in the documentation.