HUGO_ENVIRONMENT env variable is not respected, only outdated HUGO_ENV is.

[davidsneighbour]

          HUGO_ENVIRONMENT env variable is not respected, only outdated HUGO_ENV is.

Originally posted by @bubenkoff in #201 (comment)

[jmooring]

getenv "HUGO_ENV" is used throughout the code base (5 instances). Not sure why we're using an OS template function instead of one of these: hugo.Environment, hugo.IsDevelopment, hugo.IsProduction, or hugo.IsServer.

Perhaps we kept the using os.GetEnv for backwards compatibility, but that's no longer relevant due to the minimum version specified here:
https://github.com/theNewDynamic/gohugo-theme-ananke/blob/master/config/_default/hugo.toml#L1-L2

Related: #453, #625, #574

[davidsneighbour]

Also, the reasoning behind this was probably to disable robots.txt on dev-environment deploys only, so they are not indexed by accident, which can be done now by having a hugo.toml in a different environment config than production. I might remove that completely and document it accordingly.