Turbopack: Add --turbopack for next start (#77442)
Adds `--turbopack` for `next start` to match `next dev --turbopack` and
`next build --turbopack`. This is required to boot Turbopack builds
while webpack is still the default because there's some environment
variable differences.
Initially wanted to automatically detect Turbopack builds but the
problem is that we can write a marker during builds that will tell it's
a Turbopack build but then the environment is not available during
bootup. So it's a bit of a chicken-egg problem with getting the
environment to know it used Turbopack during build.
Why is this info needed at runtime? React server components has an
integration for every bundler and it needs to load the right version of
React for the bundler, that's why it's needed.
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