scripts: use -trimpath for "go build" in check-third-party

Since "garble build" first performs a Go build with -trimpath,
it helps to also use the flag with the "go build" step.
This way we get more build cache hits, as building a Go package with and
without the flag results in two separate builds.

Before:

	$ go clean -cache && time ./scripts/check-third-party.sh

	real  0m41.844s
	user  2m17.791s
	sys   0m35.440s

After:

	$ go clean -cache && time ./scripts/check-third-party.sh

	real  0m33.983s
	user  1m50.596s
	sys   0m28.499s

scripts/check-third-party.sh

@@ -72,7 +72,8 @@ for ((i = 0; i < ${#modules[@]}; i += 2)); do
 		fi
 
 		# Run "go build" first, to ensure the regular Go build works.
-		show go build "${module}/..."
+		# Use -trimpath to help reuse the cache with "garble build".
+		show go build -trimpath "${module}/..."
 
 		# Run the garble build.
 		show garble build "${module}/..."