Update docs regarding BAKE_VARIABLES config

[nmarinel]

BAKE_VARIABLES is actually meant to be a path to a YAML file, not a string of JSON.

Here is where help describes the setting as a path and here is where the parsing happens.

[nmarinel]

btw this came up because not having this set causes an error in the Kubernetes ingredient here. This probably qualifies as an issue - that _ctx.Environment.variables is completely undefined when the variables file isn't specified, but when an empty file is specified then _ctx.Environment.variables is populated with recipe data from the bake.yaml file.