Revert "Allow black to be missing in style.fish"

The root cause was that FISH_CHECK_LINT was not set. By
default, check.sh should fail if any tool is not installed, see
59b43986e9 (build_tools/style.fish: fail if formatters are not
available, 2025-07-24); like it does for rustfmt and clippy.

This reverts commit fbfd29d6d2.

See #11884

build_tools/style.fish

@@ -78,17 +78,17 @@ end
 if set -q python_files[1]
     if not type -q black
         echo
-        echo $yellow'Skipping python style checks - install `black` to style python'$normal
-    else
-        echo === Running "$green"black"$normal"
-        if set -l -q _flag_check
-            if not black --check $python_files
-                echo $red"Python files are not formatted correctly."$normal
-                exit 1
-            end
-        else
-            black $python_files
+        echo $yellow'Please install `black` to style python'$normal
+        exit 127
+    end
+    echo === Running "$green"black"$normal"
+    if set -l -q _flag_check
+        if not black --check $python_files
+            echo $red"Python files are not formatted correctly."$normal
+            exit 1
         end
+    else
+        black $python_files
     end
 end