Allow HardLinkStager to stage the root directory with 777 permissions

Context

The permissions for the staged directory's root were changed in !101 (merged) to be 0755 instead of 0777 due to constraints of buildbox-run-userchroot. This unfortunately causes other issues like:

• output files can't be written to the top level directory
• The worker is unable to create some output directories not in the input root

Due to these constraints, actions which should succeed won't work with any runner that uses the HardLinkStager. Lets come up with a plan to address this.