Decouples Astro CLI installation from the deploy action

[odaneau-astro]

We will be introducing a new setup-astro-cli GitHub Action. This PR removes Astro CLI installation steps from the deploy action.

• The cli-version input and the "Install Astro CLI" step have been removed.
• The action now expects the CLI to already be available in PATH and will fail with a clear error pointing to astronomer/setup-astro-cli if it is not.
• All README workflow examples have been updated to include an explicit astronomer/setup-astro-cli step.
• Implement intermediate environment variables to handle untrusted inputs for deploy-type and wait-time. See https://docs.github.com/en/actions/reference/security/secure-use#use-an-intermediate-environment-variable

[odaneau-astro]

Closing this PR to reopen a new one from a odaneau-astro: instead of odaneau-astro:main. See #137