ci/staging: leverage node_modules and gatsby cache on staging as well

[nadiamoe]

As mentioned in #1183 (comment), and contrary to what I intended with #1182, the gatsby cache is not being currently reused across PRs. The reason why is outlined in the GHA docs:

Workflow runs cannot restore caches created for child branches or sibling branches. For example, a cache created for the child feature-b branch would not be accessible to a workflow run triggered on the parent main branch. Similarly, a cache created for the feature-a branch with the base main would not be accessible to its sibling feature-c branch with the base main.

An easy way to fix this is to also use the cache on workflows targeting main, such as the deploy to staging job. This will also make this workflow faster.

[github-actions]

There's a version of the docs published here:

https://mdr-ci.staging.k6.io/docs/refs/pull/1188/merge

It will be deleted automatically in 30 days.

[imiric]

👍

[nadiamoe]

Merging now, crossing fingers so I didn't make any stupid syntax mistake 🤞🏻