Fix label

Add missing expected output file for cabal variant of Backpack Fail5 test

The setupAndCabalTest function runs the test twice: once with setup and
once with cabal. The setup variant uses setup-external-fail.out, but the
cabal variant needs setup-external-fail.cabal.out which was missing,
causing the test to expect empty output and fail.

Author: jappeace

cabal-testsuite/PackageTests/Backpack/Fail5/setup-external-fail.cabal.out

@@ -0,0 +1,9 @@
+# Setup configure
+Configuring consumer-0.1.0.0...
+Error:
+    The following packages are broken because other packages they depend on are missing. These broken packages must be rebuilt before they can be used.
+      planned package consumer-0.1.0.0
+          is broken due to missing package
+            framework-0.1.0.0 (has unfilled signature: App)
+              The package is installed as indefinite.
+              To use it, rebuild it in the same cabal project as the consumer so cabal can fill the signatures.

changelog.d/backpack-broken-error.md

@@ -1,7 +1,7 @@
 ---
 synopsis: "Improve Backpack broken-package error to show unfilled signatures"
 packages: [Cabal]
-pr: 11669
+prs: 11669
 ---
 
 When an indefinite Backpack package is installed separately (e.g. via