Replace curl | bash commands with safer steps

What does this MR do?

This MR updates the GitLab Runner installation docs to avoid piping curl directly into bash or sh. Instead, it instructs users to download the script, verify its checksum, and then execute it.

Why was this MR needed?

Piping scripts directly from the internet into the shell is insecure. If the script is malicious or altered, it could compromise the system. This MR ensures installation instructions follow safer practices

What's the best way to test this MR?

• Follow the updated installation instructions in a test environment.
• Confirm the script can be downloaded, verify the script using less to inspect its content before execution.

What are the relevant issue numbers?

None directly, but aligns with GitLab docs security best practices.

Related issue

#39184