There was recent work to add global values for `env`, `flags`, and `ldflags`. The global values would be merged with per-build values to generate the value used for the builds.

There are a couple issues with this:

- It's inconsistent with the existing code, which only has `default` values declared globally (there is no merging today).
- The name of the `flag` variable, caused a conflict with knative's `KO_FLAGS` environment variable (see ko-build#1317)

This PR does the following:

- Refactors the logic to use `defaultEnv`, `defaultFlags`, and `defaultLdflags`. This resolves both issues described above.
- Updates documentation

Fixes ko-build#1317