support custom distDir when set output:"export"

[zl2589]

Goals

1.Support custom distDir

Non-Goals

Background

Due to some company business requirements, I need to use multi-process builds in development and use static exports. However, when setting output to export, the build directory will be forced to be changed to .next, which will cause the same directory operation and thus cause the build to fail. When I modified the next.js source code to support custom build directories, I can correctly implement multi-process builds and exports.

Proposal

modify the "configOutDir" and ajust the function "hasCustomExportOutput"

[icyJoseph]

Just a clarification here, so you run the export "in parallel/concurrently", and they all collide at the build step, is that right?