Fix from https://github.com/buildpacks/pack/pull/1672 is breaking change

[natalieparellano]

Summary

This was discussed in Working Group and summarized in Slack here and here

At build time, we copy the application (source) directory into the build container using archive.WriteDirToTar. #1672 fixes weird permissions on this directory, but it's also a breaking change:

• if the build-time user and the runtime user are different (as recommended by the spec), and
• the host umask at build-time is less permissive resulting in e.g., 0755 permissions for newly created directories, and
• the runtime user tries to write to the application directory, THEN

applications can fail to start. This can be remedied (e.g., by setting the umask at build time to something like 0002) but this could be a breaking change for some users of pack.

Proposed fix:

• Partially revert Use OS-default permissions when creating archive root #1672 so that we don't create the weird permissions when includeRoot is provided to archive.WriteDirToTar, but instead of using default permissions for newly created directories on the host, simply use 0777 for the application directory as the least breaking change. See linked threads for other fixes discussed and why we feel that this one is the "best"

[dlion]

I'd like to pick this up :) @natalieparellano