Update build docs

Fix #1474

docs/setup.en.md

@@ -47,12 +47,12 @@ You need to generate config file via `npm run config` command.
 
 *5.* Build Misskey
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-We need to use `node-gyp` to build the `crypto` module.
 
-1. `npm install -g node-gyp`
-2. `node-gyp configure`
-3. `node-gyp build`
-4. `npm run build`
+If you're on Debian, you will need to install the `build-essential` package.
+
+Then build misskey:
+
+`npm run build`
 
 *6.* That is it.
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