update environment variable doc to reflect HAB_REFRESH_CHANNEL

Signed-off-by: Matt Wrock <[email protected]>

components/docs-chef-io/content/habitat/environment_variables.md

@@ -32,6 +32,7 @@ This is a list of all environment variables that can be used to modify the opera
 | `HAB_ORG` | Supervisor | no default | Organization to use when running with [service group encryption]({{< relref "sup_secure" >}})
 | `HAB_ORIGIN` | build system | no default | Origin used to build packages. The signing key for this origin is passed to the build system. |
 | `HAB_ORIGIN_KEYS` | build system | no default | Comma-separated list of origin keys to automatically share with the build system |
+| `HAB_REFRESH_CHANNEL` | build system | `stable` | Channel used to retrieve plan dependencies for Chef supported origins. |
 | `HAB_RING` | Supervisor | no default | The name of the ring used by the Supervisor when running with [wire encryption]({{< relref "sup_secure" >}}) |
 | `HAB_RING_KEY` | Supervisor | no default | The contents of the ring key when running with [wire encryption]({{< relref "sup_secure" >}}). Useful when running in a container. |
 | `HAB_STUDIO_SECRET_<VARIABLE>` | build system | no default | Prefix to allow environment variables into the Studio. The prefix will be removed and your variable will be passed into the Studio at build time. |