feat: adds pipeline argument

[scefali]

This PR set the SENTRY_PIPELINE env variable so we can set the user agent here: https://github.com/getsentry/sentry-cli/blob/6f9173687fdfd56810145c5c727ff76978784814/src/api.rs

This approach was taken from Github actions: https://github.com/getsentry/action-release/blob/master/src/cli.ts#L13

[lobsterkatie]

This may just be my unfamiliarity with the way in which this is being used, but the nomenclature is a little confusing to me. Don't user agents normally include the version of the thing being the agent, not the subject of the agent's actions? And in what sense is this a pipeline?

UPDATE: Took a look at the GHA link, and yes, it should be the version of the doer, i.e., the version of @sentry/webpack-plugin, not the version of the code being packed up. You can get the correct version from package.json the same way the GHA code does.