Adds Github actions for mac and ubuntu

.github/workflows/build.yml

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+name: Unity Native Audio Plugins CI 
+
+on: [push]
+
+jobs:
+  build-ubuntu:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: prepare
+      run: |
+        cmake --version
+        version=3.15
+        build=3
+        mkdir ~/temp
+        cd ~/temp
+        wget https://cmake.org/files/v$version/cmake-$version.$build-Linux-x86_64.sh
+        sudo mkdir /opt/cmake
+        sudo sh cmake-$version.$build-Linux-x86_64.sh --prefix=/opt/cmake --skip-license
+        sudo rm /usr/local/bin/cmake
+        sudo ln -s /opt/cmake/bin/cmake /usr/local/bin/cmake
+        cmake --version
+    - name: build 
+      run: |
+        cmake --version
+        ./ci/bin/build-ubuntu.sh
+
+  build-mac:
+
+    runs-on: macOS-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: build 
+      run: ./ci/bin/build-mac.sh

ci/bin/build-mac.sh

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+#!/bin/bash
+cd "$(dirname "$0")"
+
+cd ../..
+
+rm -rf build-mac
+mkdir build-mac
+cd build-mac
+
+cmake .. -G"Xcode"

ci/bin/build-ninja.sh

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+#!/bin/bash
+cd "$(dirname "$0")"
+
+cd ../..
+
+rm -rf build-ninja
+mkdir build-ninja
+cd build-ninja
+
+cmake .. -G"Ninja"

ci/bin/build-ubuntu.sh

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+#!/bin/bash
+cd "$(dirname "$0")"
+
+cd ../..
+
+rm -rf build-ubuntu
+mkdir build-ubuntu
+cd build-ubuntu
+
+cmake ..