Replace yarn alias with a shell script

This makes the yarn command available in Nix shell and direnv.

src/tmpl/yarn-project.nix.in

@@ -1,49 +1,49 @@
 # This file is generated by running "yarn install" inside your project.
 # Manual changes might be lost - proceed with caution!
 
-{ lib, nodejs, stdenv, fetchurl, writeText, git, cacert, writeShellApplication }:
+{ lib, stdenv, nodejs, git, cacert, fetchurl, writeShellScript, writeShellScriptBin }:
 { src, overrideAttrs ? null, ... } @ args:
 
 let
 
-  yarnPath = ./@@YARN_PATH@@;
-  lockfile = ./@@LOCKFILE@@;
   cacheFolder = @@CACHE_FOLDER@@;
+#@@ IF NEED_ISOLATED_BUILD_SUPPRORT
+  lockfile = ./@@LOCKFILE@@;
+#@@ ENDIF NEED_ISOLATED_BUILD_SUPPRORT
 
   # Call overrideAttrs on a derivation if a function is provided.
   optionalOverride = fn: drv:
     if fn == null then drv else drv.overrideAttrs fn;
 
+  # Simple stub that provides the global yarn command.
+  yarn = writeShellScriptBin "yarn" ''
+    exec '${nodejs}/bin/node' '${./@@YARN_PATH@@}' "$@"
+  '';
+
   # Common attributes between Yarn derivations.
   drvCommon = {
     # Make sure the build uses the right Node.js version everywhere.
-    buildInputs = [ nodejs ];
+    buildInputs = [ nodejs yarn ];
     # Tell node-gyp to use the provided Node.js headers for native code builds.
     npm_config_nodedir = nodejs;
     # Tell node-pre-gyp to never fetch binaries / always build from source.
     npm_config_build_from_source = "true";
-    # Defines the shell alias to run Yarn.
-    postHook = ''
-      yarn() {
-        CI=1 node "${yarnPath}" "$@"
-      }
-    '';
   };
 
   # Create derivations for fetching dependencies.
   cacheDrvs = let
-    builder = builtins.toFile "builder.sh" ''
+    builder = writeShellScript "yarn-cache-builder" ''
       source $stdenv/setup
       cd "$src"
       HOME="$TMP" yarn_cache_folder="$TMP" CI=1 \
-        node '${yarnPath}' nixify fetch-one $locator
+        yarn nixify fetch-one $locator
       # Because we change the cache dir, Yarn may generate a different name.
       mv "$TMP/$(sed 's/-[^-]*\.[^-]*$//' <<< "$outputFilename")"-* $out
     '';
   in lib.mapAttrs (locator: { filename, sha512 }: stdenv.mkDerivation {
     inherit src builder locator;
     name = lib.strings.sanitizeDerivationName locator;
-    buildInputs = [ nodejs git cacert ];
+    buildInputs = [ yarn git cacert ];
     outputFilename = filename;
     outputHashMode = "flat";
     outputHashAlgo = "sha512";
@@ -52,7 +52,7 @@ let
 
   # Create a shell snippet to copy dependencies from a list of derivations.
   mkCacheBuilderForDrvs = drvs:
-    writeText "collect-cache.sh" (lib.concatMapStrings (drv: ''
+    writeShellScript "collect-yarn-cache" (lib.concatMapStrings (drv: ''
       cp ${drv} '${drv.outputFilename}'
     '') drvs);
 
@@ -172,13 +172,9 @@ let
 
     passthru = {
       inherit nodejs;
-      yarn = writeShellApplication {
-        name = "yarn";
-        runtimeInputs = [ nodejs ];
-        text = ''
-          ${yarnPath} --cwd ${project}/libexec/${project.name} "$@"
-        '';
-      };
+      yarn = writeShellScriptBin "yarn" ''
+        exec '${yarn}/bin/yarn' --cwd '${project}/libexec/${project.name}' "$@"
+      '';
     };
   });