Alternate workspace with concurrent builds disabled, why?

[Simon Richter]

Hi,

I have a matrix project, with the toplevel project tied to master and
the individual builds selecting agents by label, plus other axes:

- cpu = (arm64, amd64, x86)
- build = (debug, release)
- compiler = (msvc)

where "msvc" is an agent label.

The project does no allow concurrent execution, but I see some of the
builds executing in an @2 workspace -- either all six of them, or one in
the primary workspace and five others in the @2.

The agent uses "-Dhudson.util.ProcessTree.disable=true" to allow
pdbsrv.exe to continue running after a build exits, to avoid parallel
builds terminating with an RPC error.

How can I find out why Jenkins decides to create a new workspace?

Simon