Docs helper creates changes on consecutive runs for env_vars.yaml but there are none

When running `make -C docs docs-generate` one of the possible outputs is a changed file `env_vars.yaml`.

Currently and this is bad, when running make consecutive multiple time, the file changes every time, though the content itself is correct. This makes it defacto impossible to see deltas between real changed sources.

This needs to be fixed so we can track the delta correctly.

@dragonchaser @kobergj as discussed