Add NETLIFY_SKIP_SSR_FUNCTION option to gatsby-adapter-netlify

[digitros]

I have to use the NETLIFY_SKIP_SSR_FUNCTION environment variable on my Netlify setup(because if I don't add that Netlify generates a huge SSR function and all my builds fail, I don't have any SSR page on my site tho). But, with the new version of Gatsby(5.12) Netlify installs gatsby-adapter-netlify automatically and gatsby-plugin-netlify is rebased. The adapter doesn't have support for disabling the SSR function, so I have to stay on 5.11 of gatsby until we have that option on 5.12.