Skip SVGO action if branch being PRed is in a fork owned by an organization

[jdecked]

Problem: GitHub Actions can't perform edits to a PR when the PR is from a fork owned by an organization (see github/community#6634). This means that when such a PR is opened against jdecked/twemoji, the run-svgo action fails at the commit step if SVG optimization is needed.

Proposed Solution: Skip SVGO action if branch being PRed is in a fork owned by an organization. This will probably only affect PRs from Discord, which is fine because we know the committers there.