Basically wrote the whole thing.

node_modules/@parcel/watcher/scripts/build-from-source.js

@@ -0,0 +1,13 @@
+#!/usr/bin/env node
+
+const {spawn} = require('child_process');
+
+if (process.env.npm_config_build_from_source === 'true') {
+  build();
+}
+
+function build() {
+  spawn('node-gyp', ['rebuild'], { stdio: 'inherit', shell: true }).on('exit', function (code) {
+    process.exit(code);
+  });
+}