Use self to determine if we are in browser or worker

[mlahp7]

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[AbhiPrasad]

I'm surprised it's still breaking - though we might need to more aggressively check with getGlobalObject from @sentry/utils -> https://github.com/getsentry/sentry-javascript/blob/master/packages/utils/src/misc.ts#L31.

Anyway, thank you for bringing this up, we'll get on this.

[kamilogorek]

Superseded by #3996 and merged into master, as well as covered by linter in #3999. Thanks for the PR nonetheless!