fix: use unique canonical links for product pages

[sarahdayan]

The website currently uses siteUrl (https://store.gatsbyjs.org) in the canonical link element of every page, including the product pages. This uses the page's unique pathname instead.

[jlengstorf]

Thank you!

[gatsbot]

Holy buckets, @sarahdayan — we just merged your PR to Gatsby! 💪💜

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