Use plugin to access environment variables

[meeh0w]

Overview

• Installs gatsby-plugin-env-variables plugin so we don't need to worry about proxying webportal's env variables with GATSBY_ prefix to make them available for Gatsby app.
• Related PR: https://github.com/SkynetLabs/skynet-webportal/pull/2163 - I'll make that PR ready for review once this is merged & released (skynet-webportal would break without these changes being in the image).

✅ Tested on dev3.

[linear[bot]]

SKY-981 Use a plugin for easier access to env variables

We can use a plugin to make env variables accessible for Gatsby without modifying webportal's docker-compose file, similar to what `website` repo is doing: * plugin: gatsby-plugin-env-variables * reference: [https://github.com/SkynetLabs/webportal-website-skynetlabs/blob/main/gatsby-config.js#L45-L50](https://github.com/SkynetLabs/webportal-website-skynetlabs/blob/main/gatsby-config.js#L45-L50)

[github-actions[bot]]

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