[cmd/builder] Disable CGO by default

[evan-bradley]

Is your feature request related to a problem? Please describe.
When using a binary output by the builder with no options, I can't use it by default in a scratch image unless it is compiled with CGO_ENALBED=0.

Describe the solution you'd like
Turn off CGO by default in the builder. Allow users to enable it through setting the CGO_ENABLED environment variable.

Additional context
Our contributing guidelines forbid using CGO, and no upstream components I'm aware of use it. We also have CGO_ENABLED=0 set anywhere we build Collector binaries in upstream repositories.

Users have also run into this before: #6373

[Red-GV]

At the very minimum, can this be documented somewhere? Lack of this actually makes it somewhat difficult to follow the tutorial because without this setting disabled, the binary doesn't work.

[axw]

I started looking at documenting it, but figured I may as well make the code change to disable it by default at the same time: #14094