Use pip3 to get ninja

.github/workflows/main.yml

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+name: CI
+
+on: [push]
+
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-18.04, ubuntu-20.04, macos-10.15, windows-2019]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Get Ninja
+        id: ninja
+        uses: ./ # Uses an action in the root directory
+
+      - name: Did we get Ninja?
+        run: echo "Ninja version was '${{ steps.ninja.outputs.version }}'"

README.md

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+# Get Ninja GitHub Action
+
+Gets the most recent Ninja
+
+**Works on**: Linux, Windows and MacOS 
+
+## Inputs
+
+No inputs
+
+## Outputs
+
+### `version`
+
+The version string from "ninja --version"
+
+## Example usage
+
+~~~~
+    - name: Install Ninja
+      id: ninja
+      uses: turtlesec-no/get-ninja@main
+
+    - name: Ninja version
+      run: echo "${{ steps.ninja.outputs.version }}"
+~~~~

action.yml

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+name: 'Get Ninja'
+description: 'Installs Ninja'
+author: 'TurtleSec'
+outputs:
+  version:
+    description: "The version of Ninja which was installed"
+    value: ${{ steps.output_version.outputs.version }}
+runs:
+  using: "composite"
+  steps:
+    - name: Install on Mac/Linux/Windows
+      run: |
+        pip3 install wheel setuptools
+        pip3 install ninja --upgrade
+      shell: bash
+    - name: Fix path on Linux
+      run: |
+        echo "/home/runner/.local/bin" >> $GITHUB_PATH
+      shell: bash
+    - name: Fill in output variable
+      id: output_version
+      shell: bash
+      run: |
+        ninja --version
+        echo "::set-output name=version::$(ninja --version)"
+branding:
+  icon: "archive"
+  color: "green"