riscv64: disable gtests for unsupported components.

Bug: b/271573990

Test: gtests on host:
  lunch aosp_riscv64-userdebug && m test-art-host-gtest

Test: gtests on target (on a Linux RISC-V VM):
  lunch aosp_riscv64-userdebug

  export ART_TEST_SSH_USER=ubuntu
  export ART_TEST_SSH_HOST=localhost
  export ART_TEST_SSH_PORT=10001
  export ART_TEST_ON_VM=true

  . art/tools/buildbot-utils.sh
  art/tools/buildbot-build.sh --target

  # Create, boot and configure the VM.
  art/tools/buildbot-vm.sh create
  art/tools/buildbot-vm.sh boot
  art/tools/buildbot-vm.sh setup-ssh  # password: 'ubuntu'

  art/tools/buildbot-cleanup-device.sh
  art/tools/buildbot-setup-device.sh
  art/tools/buildbot-sync.sh

  art/tools/run-gtests.sh

Change-Id: I278e3453406a91a5e9d03645cafb9a9d1f82d896
diff --git a/tools/run-gtests.sh b/tools/run-gtests.sh
index da61c7e..99ac8df 100755
--- a/tools/run-gtests.sh
+++ b/tools/run-gtests.sh
@@ -61,7 +61,7 @@
 
 run_in_chroot() {
   if [ -n "$ART_TEST_ON_VM" ]; then
-    $ART_SSH_CMD $ART_CHROOT_CMD $@
+    $ART_SSH_CMD $ART_CHROOT_CMD env ANDROID_ROOT=/system $@
   else
     "$adb" shell chroot "$ART_TEST_CHROOT" $@
   fi