Add unzip to requirements

When running Step #3 `flutter channel dev` got:
```Missing "unzip" tool. Unable to extract Dart SDK.
Consider running "sudo apt-get install unzip".```

doc/BUILD_ON_LINUX.md

@@ -23,7 +23,7 @@ Note:
 ### Detail steps
 1. Install prerequisites 
 ```shell
-sudo apt-get install curl build-essential libsqlite3-dev libssl-dev clang cmake ninja-build pkg-config libgtk-3-dev
+sudo apt-get install curl build-essential libsqlite3-dev libssl-dev clang cmake ninja-build pkg-config libgtk-3-dev unzip
 # optional, for generating protobuf in step 8 only
 sudo apt-get install protobuf-compiler
 ```