Directly query environment platform in KernelSetArgSampler backend check.

The fixture's platform is initialized to point at the environment one in
the platform test SetUp(), but that's too late for this query. This is a
little hacky but it lets us skip the test without building the kernel
(which will potentially cause a fail if images aren't supported).

Author: aarongreig

test/conformance/kernel/urKernelSetArgSampler.cpp

@@ -9,8 +9,9 @@ struct urKernelSetArgSamplerTest : uur::urKernelTest {
     void SetUp() {
         // Images and samplers are not available on AMD
         ur_platform_backend_t backend;
-        ASSERT_SUCCESS(urPlatformGetInfo(platform, UR_PLATFORM_INFO_BACKEND,
-                                         sizeof(backend), &backend, nullptr));
+        ASSERT_SUCCESS(urPlatformGetInfo(
+            uur::PlatformEnvironment::instance->platform,
+            UR_PLATFORM_INFO_BACKEND, sizeof(backend), &backend, nullptr));
         if (backend == UR_PLATFORM_BACKEND_HIP) {
             GTEST_SKIP() << "Sampler are not supported on hip.";
         }