Make CGO_ENABLED Configurable

[ArkaSaha30]

What would you like to be added?

The value of CGO_ENABLED is hard-coded to 0. I would want to make CGO_ENABLED optionally configurable by passing an environment variable during runtime. When no environment variable is passed it will default to 0 therefore keeping the build process intact as defined in the script.

Ref: #17418

Why is this needed?

CGO_ENABLED is an important Environment Variable required during Go compilation and build process. Making it configurable will allow consumers to build etcd as per their requirements, specifically I have come across FIPS compilation requirement which requires the CGO_ENABLED=1. The suggested PR will not hinder the existing build process but allow users to configure CGO_ENABLED environment variable as per their requirement.

[ahrtr]

@ArkaSaha30 Please add a changelog for both 3.5 and 3.4. thx

[ahrtr]

All done.