build: fix yarn errors upon npm install minidump

- Only run `git submodule update` when running out of a git working copy
- Don't require `yarn` for end-user installation

build.js

@@ -27,6 +27,14 @@ function spawnSync (...args) {
   }
 }
 
+if (fs.existsSync(path.join(__dirname, '.git'))) {
+  // this is a development working tree of `node-minidump`, not an end-user install of the `minidump` package
+  spawnSync('git', ['submodule', 'update', '--init', '--recursive'], {
+    cwd: __dirname,
+    stdio: 'inherit'
+  })
+}
+
 const buildDir = path.join(__dirname, 'build', getEffectiveArch())
 if (!fs.existsSync(buildDir)) {
   fs.mkdirSync(buildDir, { recursive: true })

package.json

@@ -17,7 +17,7 @@
   },
   "scripts": {
     "prepublishOnly": "shx chmod -R +x ./bin",
-    "preinstall": "yarn submodule && node build.js",
+    "preinstall": "node build.js",
     "submodule": "git submodule update --init --recursive",
     "test": "mocha test && standard"
   },