fix(core-server): build config inconsistency

[lihbr]

Issue: #14561 (more info there), nuxt-community/storybook#259

buildDev() and buildStatic() don't resolve the configDir option the same way:

• buildDev() configuration resolver: https://github.com/storybookjs/storybook/blob/next/lib/core-server/src/build-dev.ts#L122
• buildStatic() configuration resolver: https://github.com/storybookjs/storybook/blob/next/lib/core-server/src/build-static.ts#L110

What I did

Made configDir resolve strategy the same as in: https://github.com/storybookjs/storybook/blob/next/lib/core-server/src/build-static.ts#L110

How to test

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

Additional context

Was unable to run tests on Windows because they seem to rely on POSIX paths where my machine uses win32 ones. However I checked before/after and consistently have the same amount of test "failing" because of that.

cc @ndelangen

[shilman]

Thanks @lihbr !!!