[wip experiment] Key transient MSBuild Server per build root, not one-per-machine

[JanProvaznik]

Context

A /mt build with node reuse off (-nr:false, as dotnet restore does) engages the MSBuild Server purely to obtain Server GC, which /mt depends on for performance, and then shuts the server down after the build (added in #14248).

But the server's pipe and mutex names are one-per-machine (keyed by architecture/user/version/session). So when two such transient builds run at the same time, they contend for a single server: all but one find it busy and fall back to an in-process, Workstation-GC build — defeating the whole point of engaging the server under /mt.

Change

Key transient (non-reused) servers per build root instead of one-per-machine:

• CanRunServerBasedOnCommandLineSwitches now emits a serverRootKey — the entry project's full path, or the current directory when no project is resolved — whenever shutdownServerAfterBuild is set (/mt + node reuse off).
• Main folds that key into the MSBUILDNODEHANDSHAKESALT before handing off to the client. The salt drives the handshake hash that names the pipe and mutex, and the launched server inherits the client's environment, so both derive the same per-root names.
• Parallel builds of different roots each get their own short-lived Server-GC server. Builds of the same root still share one server (the second concurrent build falls back), so it is "one transient server per root", not an unbounded number per invocation.
• Reusable (non-transient) servers are unchanged and remain one-per-machine.

Testing

New unit test TransientServerIsKeyedPerRootAllowingConcurrentServers: two concurrent distinct-root -mt -nr:false builds run in two distinct, simultaneously-alive server processes, both with Server GC, and both shut down afterward. All existing MSBuildServer_Tests pass.

Verified live on the bootstrap:

• 3 / 5 distinct roots in parallel → 3 / 5 distinct concurrent /nodemode:8 servers, each ServerGC=True, 0 survivors afterward.
• Same-root contrast (3 clients, 1 root) → exactly 1 server (ServerGC=True); the other two fall back to in-proc Workstation-GC builds.
• Real dotnet build muxer (DOTNET_CLI_USE_MSBUILD_SERVER=1), 2 roots → two distinct MSBuild.exe /nodemode:8 Server-GC servers concurrently, both transient.

Docs updated in documentation/MSBuild-Server.md.