cmd/cue: mention --inject-vars/-T flag for injection help

Otherwise it's not clear the flag needs to be set to enable injection
vars

Signed-off-by: Paul Jolly <[email protected]>
Change-Id: Iffd8fad2eaec1e990881926242926f87bfe8be7a
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211222
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: myitcv

cmd/cue/cmd/help.go

@@ -731,7 +731,8 @@ ensures the user may only specify "prod" or "staging".
 Tag variables
 
 The injection mechanism allows for the injection of system variables:
-when variable injection is enabled, tags of the form
+when variable injection is enabled by passing the --inject-vars/-T flag,
+tags of the form
 
     @tag(dir,var=cwd)