Update brew install instructions to include required --cask (#61890)

Author: rfearing

content/copilot/how-tos/copilot-cli/cli-getting-started.md

@@ -42,7 +42,7 @@ Use one of these commands:
 * **macOS/Linux (Homebrew)**
 
   ```bash copy
-  brew install copilot-cli
+  brew install --cask copilot-cli
   ```
 
 ## Starting the CLI for the first time

content/copilot/how-tos/copilot-cli/set-up-copilot-cli/install-copilot-cli.md

@@ -72,13 +72,13 @@ winget install GitHub.Copilot.Prerelease
 ### Installing with Homebrew (macOS and Linux)
 
 ```shell copy
-brew install copilot-cli
+brew install --cask copilot-cli
 ```
 
 To install the prerelease version:
 
 ```shell copy
-brew install copilot-cli@prerelease
+brew install --cask copilot-cli@prerelease
 ```
 
 ### Installing with the install script (macOS and Linux)