title: can't install a python package & use it in a GitHub Action
body: <p>I have a <a href="https://github.com/dtenenba/actions-test" rel="nofollow noreferrer">GitHub Repo</a> with a simple GitHub Actions workflow:</p>
  
  <pre><code>name: CI
  
  on: [push]
  
  jobs:
    build:
  
      runs-on: ubuntu-latest
  
      steps:
      - uses: actions/checkout@v1
      - name: Run a one-line script
        run: echo Hello, world!
      - name: Run a multi-line script
        run: |
          echo Add other actions to build,
          echo test, and deploy your project.
          pip install twine
          twine --help
  </code></pre>
  
  <p>When I run it, it installs twine, but <a href="https://github.com/dtenenba/actions-test/commit/e554f922b8125cbc662cc9b927658dd51ffe4a7f/checks?check_suite_id=345028062" rel="nofollow noreferrer">fails</a> to run it, saying the command is not found:</p>
  
  <pre><code>...
  Successfully installed Pygments-2.5.2 bleach-3.1.0 certifi-2019.11.28 chardet-3.0.4 docutils-0.15.2 idna-2.8 pkginfo-1.5.0.1 readme-renderer-24.0 requests-2.22.0 requests-toolbelt-0.9.1 setuptools-42.0.2 six-1.13.0 tqdm-4.40.0 twine-1.15.0 urllib3-1.25.7 webencodings-0.5.1
  /home/runner/work/_temp/1643cb1d-8b12-4aa8-8e1d-bd5bba60fd5b.sh: line 4: twine: command not found
  </code></pre>
  
  <p>How can I get this to work?</p>
  
  <p>Thanks in advance. </p>
  
  <p>I am already aware of existing GitHub actions that use twine, they don't do what I want. I also know that I can fork an action and modify it, but what I really want to know is, if I can run <code>echo hello world</code> why can't I run <code>pip install some-pkg; some-pkg ...</code> ?</p>
  
  <p>It does not seem to be a path problem. I've tried using <code>python -m twine</code> in place of <code>twine</code>. I tried running a <code>find</code> command to find all files on the machine that have <code>twine</code> in their name (it didn't find any). </p>