[BUG] Cannot override `.npmrc` env var `PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD` from true to false

Context:

• Playwright Version: 1.5.1
• Operating System: Linux for CI, Mac locally
• Node.js version: 12.16.2
• Browser: All

Describe the bug

The scenario we have is that we want to skip installing browsers by default in our ecosystem, but manually install browsers if a person is running playwright code locally or as part of the CI job that runs playwright code. As a result we set the following in our .npmrc:

playwright_skip_browser_download=1

Unfortunately, PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD="" yarn does not override this value (or any variation of this). I think this is because the concept of overriding env vars in the playwright repo is predicated on the idea of overriding falsey values with truthy values and not the other way around.

If we look at the code to skip the browser install:

  if (getFromENV('PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD')) {
    browserFetcher.logPolitely('Skipping browsers download because `PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD` env variable is set');
    return false;
  }
``
We need `getFromENV('PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD')` to return a falsey value to get past this block. When we look at the code from [`getFromEnv`](https://github.com/microsoft/playwright/blob/master/src/utils/utils.ts#L98-L103):

export function getFromENV(name: string) { let value = process.env[name]; value = value || process.env[npm_config_${name.toLowerCase()}]; value = value || process.env[npm_package_config_${name.toLowerCase()}]; return value; }

we see that it's impossible to override the truthy value of `process.env.npm_config_playwright_skip_browser_download` with a falsey value no matter what we do. I think changes the `||` operators to `??` might fix the issue.

Additionally, it might be worth being explicit about possible values for `PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD`, cast them from a string to a number (`"0"` is true after all, while `0` is not) and do a !! in the check. 

I'll put up a corresponding PR for your review.

[pavelfeldman]

Btw, for installing the browsers, you could also run npx playwright-cli

[majapw]

Thank you!