environment_variables.md: Move ext dep link back to proper section

docs/usability/environment_variables.md

@@ -27,6 +27,8 @@ Web dependencies. After the initial fetch, this can greatly improve the performa
 calling `stuart_update` and make it far easier to switch between multiple branches or
 scopes where dependencies may change.
 
+For more info, see [the External Dependencies docs](using_extdep.md).
+
 ## EDK_BUILD_CMD
 
 If present, the absolute path to an application to use for the edk build process will be
@@ -36,5 +38,3 @@ invoked instead of `build`. This is primarily used to allow a tool to wrap aroun
 
 If present, these parameters will be passed to the build command. This is primarily used to
 pair wrapper-specific parameters with the wrapper passed in `EDK_BUILD_CMD`.
-
-For more info, see [the External Dependencies docs](using_extdep.md).