v2: use XP_CHANNEL=preview for CLI install

[jbw976]

This PR just updates the CLI installation instructions in the v2.0-preview docs to always use XP_CHANNEL=preview. This way, when using v2, folks will download and install the Crossplane CLI from the v2 preview.

The command in action:

❯ curl -sL "https://raw.githubusercontent.com/crossplane/crossplane/main/install.sh" | XP_CHANNEL=preview sh
crossplane CLI downloaded successfully! Run the following commands to finish installing it:

sudo mv crossplane /usr/local/bin
crossplane --help

Visit https://crossplane.io to get started. 🚀
Have a nice day! 👋

❯ sudo mv crossplane /usr/local/bin

This gives me the latest version from the v2 preview:

❯ crossplane version
Client Version: v2.0.0-preview.0.39.g815fa2bba
...

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[negz]

Good catch!