add .yarnrc env config option

__tests__/fixtures/lifecycle-scripts/yarnrc-env/.yarnrc

@@ -0,0 +1,2 @@
+env:
+  FOO "BAR"

__tests__/fixtures/lifecycle-scripts/yarnrc-env/log-command.js

@@ -0,0 +1 @@
+console.log(process.env.FOO);

__tests__/fixtures/lifecycle-scripts/yarnrc-env/package.json

@@ -0,0 +1,9 @@
+{
+  "name": "yarn-config-env",
+  "version": "1.0.0",
+  "main": "index.js",
+  "license": "MIT",
+  "scripts": {
+    "preinstall" : "node log-command.js"
+  }
+}

__tests__/lifecycle-scripts.js

@@ -112,3 +112,8 @@ test('should run both prepublish and prepare when installing, but not prepublish
 
   expect(stdout).not.toMatch(/^running the prepublishOnly hook$/m);
 });
+
+test('should allow setting environment variables via yarnrc', async () => {
+  const stdout = await execCommand('install', 'yarnrc-env');
+  expect(stdout).toMatch(/^BAR$/m);
+});

src/util/execute-lifecycle-script.js

@@ -28,6 +28,9 @@ async function makeEnv(stage: string, cwd: string, config: Config): {
 } {
   const env = Object.assign({}, process.env);
 
+  // Merge in the `env` object specified in .yarnrc
+  Object.assign(env, config.getOption('env') || {});
+
   env.npm_lifecycle_event = stage;
   env.npm_node_execpath = env.NODE || process.execPath;
   env.npm_execpath = env.npm_execpath || process.mainModule.filename;