[nightly] depending on pipeline execution we sometimes refer to the wrong workspace

we set BASEDIR at the top of our pipeline to point at the component checkout within WORKSPACE.

but!

a) at that point WORKSPACE is the workspace for the launching task
b) sometimes our parallel executions get a task with a different local WORKSPACE to allow for coexisting on the same build host
c) when this happens our parallel stages are referring to some other absolute path on the host
d) in most cases we're referring to dev-support files like e.g. the nightly build script or machine info script that are the same across branches so things are fine if we aren't running at the same time as a job that's overwritting them
e) we also refer to the Dockerfile this way, so weird bugs I'm sure.
f) we build the source tarball from here, so that's probably broken subtly
g) sometimes that other directory doesn't exist at all and we fail with confusing messages about stuff not found