internal: Deny the device tree on the hosts that have one

[Yos-X]

On a host without a device tree the floor cannot be built, so the agent plugin is not created and everything that starts the runtime fails.

What happens

standardRoots denies Path.of("/dev") on every host. On Windows that path is \dev, which names no root of its own — Files.isAbsolute is false — so canonicalizing it is refused:

java.lang.IllegalArgumentException: A path to canonicalize is absolute and of the default file system, was '\dev'.

AgentFloor is built from those roots, so a suite that starts the runtime fails before a test runs:

Cannot start runtime 'harness': stage GRAPH failed (1 problem):
- Cannot create org.foedusprogramme.alexandrite.agent.permission.AgentFloor with
  org.foedusprogramme.alexandrite.agent.permission.AgentFloor (plugin alexandrite-agent):
  java.lang.IllegalArgumentException: A path to canonicalize is absolute and of the default file system, was '\dev'.

In the checkout this was found in, 265 of the 658 tests of :libraries:agent:test fail this way. The message names the floor rather than the platform, so the cause is not visible from the failures themselves.

The change

The device tree is a root where there is one, so it is denied where the host separates paths with / and nowhere else. Nothing else of standardRoots changes: the credential stores are under the home directory, and the moved stores come from the environment.

Verification

• :libraries:internal:compileKotlin and :libraries:internal:spotlessCheck pass on this branch.
• On the branch this was found in, the 265 failures that could not start the runtime are gone with the change and return without it.

Found while adding a channel for OneBot. The floor is not that channel's, but no channel can run the agent plugin on such a host until this is fixed.

[Yos-X]

Superseded by the branch that now holds every fix of this kind: \ix/platform, which carries this fix and the write-permission one, both rebased onto the current \master. The commit that was here is the first of them.