Add option to make npm install optional

[luisherranz]

First of all, thanks for the action. It's awesome 🙂

We had a problem using it, though. We use a lerna monorepo and the npm install that you do inside the docker container fails, so I've added an option to avoid running npm install in the container and thus allow people to run their necessary install and/or build scripts in a previous step of their workflow, like this:

jobs:
  eslint:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout Repo
        uses: actions/checkout@v2
      - name: Install dependencies
        run: npm ci
      - name: Run a build step
        run: npm run build
      - uses: bradennapier/eslint-plus-action@v2
        with:
          github-token: ${{secrets.GITHUB_TOKEN}}
          npmInstall: false

https://github.com/frontity/frontity/blob/refactor-analytics-event/.github/workflows/eslint.yml

The option is called npmInstall and has a default of true.

Let me know what you think.

[bradennapier]

Yeah it makes sense that this is an issue - my goal was to reduce the steps needed to get this working but it makes sense to set this up better - going to see if there may be a more automated way of doing it.

[bradennapier]

Let me know your thoughts on the suggested changes. We should probably also update the readme as-to how this would work.

[bradennapier]

Ok will merge these in, however I am going to see about waiting until this major release I am working on that should make it possible to lint remote forks securely. I will let you know when released (as v3 most likely).

see: #11

[luisherranz]

No problem. I just subscribed to the repo 🙂