install-bindist: Test for manpage installation

See ghc#22371 (closed).

.gitlab/install-bindist.sh

@@ -123,6 +123,7 @@ case "$(uname)" in
   Linux)
     if [ -z "$BINDIST_ONLY" ]; then
         if ! ( man 1 ghc >/dev/null ); then
+            man 1 ghc
             echo "Failed to find ghc(1) manpage"
             exit 1
         fi