swift: Workaround Hydra darwin build problem

Fail to build with ninja 1.12 when NIX_BUILD_CORES is low (Hydra or Github Actions):

```
ld: warning: directory not found for option '-L/nix/store/g9rbp9m6vs1xj4jl6b6vjb6bm8kgr107-SDKs/MacOSX10.15.sdk/usr/lib/swift'
...
ld: warning: Could not find or use auto-linked library 'swiftCompatibility56'
Undefined symbols for architecture arm64:
  "__swift_FORCE_LOAD_$_swiftCompatibility56", referenced from:
      __swift_FORCE_LOAD_$_swiftCompatibility56_$_Optimizer in libswiftCompilerModules-bootstrapping1.a(Optimizer.o)
  ...
```

Can reproduce using `nix --option cores 2 build -f . swiftPackages.swift-unwrapped`.

Until we find out the exact cause, follow [swift upstream][1], pin ninja to version
1.11.1.

[1]: swiftlang/swift#72989

pkgs/development/compilers/swift/compiler/default.nix

@@ -6,7 +6,7 @@
 , coreutils
 , gnugrep
 , perl
-, ninja
+, ninja_1_11
 , pkg-config
 , clang
 , bintools
@@ -190,6 +190,14 @@ let
     '';
   };
 
+  # https://github.com/NixOS/nixpkgs/issues/327836
+  # Fail to build with ninja 1.12 when NIX_BUILD_CORES is low (Hydra or Github Actions).
+  # Can reproduce using `nix --option cores 2 build -f . swiftPackages.swift-unwrapped`.
+  # Until we find out the exact cause, follow [swift upstream][1], pin ninja to version
+  # 1.11.1.
+  # [1]: https://github.com/swiftlang/swift/pull/72989
+  ninja = ninja_1_11;
+
 in stdenv.mkDerivation {
   pname = "swift";
   inherit (sources) version;