remove drafted content from Gatsby Cloud builds

[thinkybeast]

Currently, drafted records in DatoCMS appear in both Gatsby Cloud deploy builds and previews.

To reproduce:

• set up a Dato CMS starter site in Gatsby Cloud
• In the DatoCMS admin, go to Settings > Models > Work > Settings > Additional settings > Enable draft/published system Create a New Record > Save
• The new record should be saved as a draft
• Trigger a new build

Actual: The drafted record appears as a page in the deploy build and the preview build. Expected: The drafted record only appears in the preview build.

This change checks that this is both a Gatsby Cloud build, and that this is specifically a Preview build (the executing command is gatsby develop.

[stefanoverna]

Nice! Released in v2.5.2, thanks @thinkybeast !