test(desktop): provide the required primary runtime directory in the welcome startup spec

`welcome-startup.spec.ts` imports the unpackaged main entry, which now
requires `DSH_DESKTOP_PRIMARY_RUNTIME_DIR`. Stub it beside the neighbouring
`DSH_DESKTOP_*` values so the suite reaches the welcome startup it covers.

apps/desktop/tests/welcome-startup.spec.ts

@@ -148,6 +148,7 @@ it('starts the Host for welcome onboarding and opens the workspace on skip witho
   vi.stubEnv('DSH_DESKTOP_NODE_BINARY', '/runtime/node')
   vi.stubEnv('DSH_DESKTOP_PNPM_ENTRY', '/runtime/pnpm')
   vi.stubEnv('DSH_DESKTOP_DSH_DIR', '/runtime/dsh')
+  vi.stubEnv('DSH_DESKTOP_PRIMARY_RUNTIME_DIR', '/runtime/primary-runtime')
   vi.stubEnv('DSH_DESKTOP_HOST_INSPECT_PORT', undefined)
   vi.stubEnv('DSH_DESKTOP_OPEN_DEVTOOLS', '0')
   vi.stubEnv('DSH_DESKTOP_MANDATORY_UPDATE_CONFIG', undefined)