fix(tests): disable MSBuild node reuse for processes spawned by the test suite

ExecProcessWithLambdas (FakeLib 4.64.17) ends with an unbounded WaitForExit() that the
7-minute timeout above it does not cover:

  IL_020b:  WaitForExit(int32)   // the timeout
  IL_0217:  Kill()               // "Process %s %s timed out."
  IL_02b0:  WaitForExit()        // unbounded, after
  IL_02b6:  get_ExitCode()

That call returns only once the redirected streams reach EOF. With node reuse on,
dotnet build/run/pack leave MSBuild worker nodes behind holding the inherited stdout
and stderr handles, so EOF never comes and the run hangs with no output -- which is
why PROCESS TIMED OUT never appeared in the log.

ConditionSpecs is the only fixture that nests dotnet run and dotnet pack, and it is
where the Windows run stops.

Author: fpellet

integrationtests/Paket.IntegrationTests/TestHelper.fs

@@ -139,6 +139,11 @@ let directToolEx env isPaket toolInfo commands workingDir =
     #else
     Environment.SetEnvironmentVariable("PAKET_DETAILED_ERRORS", "true")
     Environment.SetEnvironmentVariable("PAKET_DETAILED_WARNINGS", "true")
+    // ExecProcessWithLambdas ends with an unbounded WaitForExit(), which returns only once the
+    // redirected streams reach EOF -- the timeout below does not cover it. With node reuse on,
+    // `dotnet build/run/pack` leaves MSBuild worker nodes behind that inherit those handles, so
+    // EOF never comes and the run hangs silently. Children inherit this variable.
+    Environment.SetEnvironmentVariable("MSBUILDDISABLENODEREUSE", "1")
     printfn "%s> %s %s" workingDir (if isPaket then "paket" else processFilename) processArgs
     let perfMessages = ResizeArray()
     let msgs = ResizeArray<OutputMsg>()