Fix next/image config injection during static generation (#47007)
Fixes #39330.
I've not added a new test, but extended the existing test instead (and
re-enabled it).
It seems to me that a custom next/image config was not considered during
static generation (which itself uses `next export`?) before. Adding the
config to `renderOpts` in `packages/next/src/export/index.ts` resolves
the issue.