Ensure process.env.TURBOPACK is always set (#6116)
### Description
Related to https://github.com/vercel/next.js/pull/56496. Server Actions
currently shows an error as it seems to use the react-dom-webpack
version instead of react-dom-turbopack. This seems to be caused by the
changes in https://github.com/vercel/next.js/pull/56438 which checks for
`process.env.TURBOPACK` which is not always set currently, unlike
`process.turbopack`. This PR adds it in the other places where
`process.turbopack` is set.
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