title: GitHub Actions - Composer failing because of PHP Version constraint
body: <p>I'm trying to get my unit-tests to run via GitHub Actions on a pull_request.</p>
  <p>I can see the actions running when I update my PR but the PHP version doesn't update when it tries to composer install my project.</p>
  <pre class="lang-sh prettyprint-override"><code>Command: composer install --no-interaction --no-suggest --no-progress
  Loading composer repositories with package information
  Installing dependencies (including require-dev) from lock file
  
  Your requirements could not be resolved to an installable set of packages.
  
    Problem 1
      - This package requires php ~7.3.0 but your PHP version (7.4.10) does not satisfy that requirement.
  </code></pre>
  <p>I've tried multiple different workflow files, and this was the one that gave me the most control, but it still seems to be running on php 7.4.</p>
  <pre class="lang-yaml prettyprint-override"><code>name: Run Tests
  
  on:
    pull_request:
      branches:
        - master
  
  jobs:
    build:
      runs-on: ${{ matrix.operating-system }}
      strategy:
        matrix:
          operating-system: [ubuntu-latest]
          php-versions: ['7.3']
  
      steps:
        # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
        - uses: actions/checkout@v2
        - uses: shivammathur/setup-php@v2
          with:
            php-version: ${{ matrix.php-versions }}
        - uses: php-actions/composer@v2
        - uses: php-actions/phpunit@v8
  </code></pre>
  <p>Finally, here is the snippet from my composer json that shows the constraint:</p>
  <pre class="lang-json prettyprint-override"><code>&quot;require&quot;: {
      &quot;php&quot;: &quot;~7.3.0&quot;,
      &quot;guzzlehttp/guzzle&quot;: &quot;^7.0&quot;
  },
  &quot;require-dev&quot;: {
      &quot;phpunit/phpunit&quot;: &quot;^8&quot;
  }
  </code></pre>