Vite: Support environment variables in viteFinal define config

[IanVS]

Issue: Ref: https://github.com/jcollicoat/vite-storybook/issues/1

What I did

@jcollicoat discovered that setting environment variables with vite's define option inside of viteFinal had no effect. I realized this was because we were setting a define rule for import.meta.env (setting it to a stringified object of the env variables we know about before handling viteFinal), so instead of the full string replacement of import.meta.env.STORYBOOK_FOO, vite was finding import.meta.env, turning it into an object, then trying to read the FOO property, which isn't there.

This is solved by moving the call to stringifyProcessEnvs after we get viteFinal (which is actually what we do in 6.5: https://github.com/storybookjs/builder-vite/blob/5f3da981f9ac89f871afaacc052f1551c4fbdde9/packages/builder-vite/build.ts#L31-L34). Note that destructuring env vars is not supported by the webpack builder.

I also removed the fallback for XSTORYBOOK_EXAMPLE_APP which was added back when we used that variable in other places, but we no longer do, so there's no need to set it.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

Should I add a sandbox story to test environment variables, similar to what we have in 6.5? (https://github.com/storybookjs/builder-vite/blob/5f3da981f9ac89f871afaacc052f1551c4fbdde9/examples/react/stories/EnvironmentVariables.stories.jsx)