install-bindist: Test for manpage installation

See ghc#22371 (closed).

.gitlab/install-bindist.sh

@@ -121,10 +121,11 @@ fi
 # Check for existence of manpage.
 case "$(uname)" in
   Linux)
-    man 1 ghc | cat
-    if ! ( man 1 ghc | grep -q "$VERSION" ); then
-        echo "Failed to find ghc(1) manpage"
-        exit 1
+    if [ -z "$BINDIST_ONLY" ]; then
+        if ! ( man 1 ghc >/dev/null ); then
+            echo "Failed to find ghc(1) manpage"
+            exit 1
+        fi
     fi
     ;;
   *)