Switch from curl | sh install to a more direct install

[Fryguy]

curl | sh is generally considered insecure, and some vendors refuse to use anything that might have a curl | sh in it. As such, we can switch to writing our own download code.

The official install scripts from which we can take inspiration:

• https://clis.cloud.ibm.com/install/powershell
• https://clis.cloud.ibm.com/install/linux
• https://clis.cloud.ibm.com/install/osx

Also https://github.com/IBM-Cloud/ibm-cloud-cli-release