Add Github action to build PRs (#1048)

# every push to a branch: build binary

name: Build

on:

  pull_request:

    types: [opened, synchronize]

jobs:

  build_binary:

    name: Build nakama binary

    runs-on: ubuntu-latest

    steps:

      - name: Setup Go

        uses: actions/setup-go@v4

        with:

          go-version: "stable"

      - name: Checkout

        uses: actions/checkout@v3

      - name: Build binary

        run: go build -trimpath -mod=vendor