docs/howto: fix tag variable page comparator

This fixes a page comparator that was missing a multi-line flag.

This caused CI to fail when the page's commands were re-run with the
cache being skipped, which only happens post-submission and not during
CL tests.

Signed-off-by: Jonathan Matthews <github@hello.jonathanmatthews.com>
Change-Id: I0cd6125005872b9c135386223f830f8e547817f9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1198563
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>

content/docs/howto/inject-system-information-into-evaluation-using-tag-variable/page.cue

@@ -2,11 +2,11 @@ package site
 
 content: docs: howto: "inject-system-information-into-evaluation-using-tag-variable": {
 	page: comparators: [
-		for cmd in ["export", "eval"] {
+		for prefix in ["cue export -T", "cue eval -T", "cue eval -Tt"] {
 			{
 				kind:          "patternComparator"
-				commandPrefix: "cue \(cmd) -T"
-				pattern: expr: #"^[a-zA-Z0-9]+: *"?(.*)"?"#
+				commandPrefix: prefix
+				pattern: expr: #"(?m)^[a-zA-Z0-9]+: *"?(.*)"?"#
 			}
 		},
 	]