Bug 1492362: [taskgraph] Only use absolute paths for checkouts in docker; r=glandium

We define a value run.workdir to point to the defaut directory to use for
all sorts of paths, including checkouts, in the job transform. However, that
directory only makes sense for docker-worker tasks, and relative paths should
be used elsewhere. Adjust the paths on non-docker-worker linux tasks to match
macOS paths.

Differential Revision: https://phabricator.services.mozilla.com/D86668