nvapi-adapter: Improve DXVK NVAPI hack detection

When DXVK_ENABLE_NVAPI is set, we can be sure that the
NVAPI hack is disabled.

src/sysinfo/nvapi_adapter.cpp

@@ -72,7 +72,8 @@ namespace dxvk {
         if (!HasNvProprietaryDriver() && allowOtherDrivers.empty())
             return false;
 
-        if (HasNvProprietaryDriver() && m_dxgiDesc.VendorId != NvidiaPciVendorId)
+        auto dxvkEnableNvapi = env::getEnvVariable("DXVK_ENABLE_NVAPI");
+        if (HasNvProprietaryDriver() && m_dxgiDesc.VendorId != NvidiaPciVendorId && dxvkEnableNvapi.empty())
             return false; // DXVK NVAPI-hack is enabled, skip this adapter
 
         if (HasNvProprietaryDriver())

tests/nvapi_sysinfo.cpp

@@ -43,6 +43,8 @@ TEST_CASE("Initialize succeeds", "[.sysinfo]") {
     }
 
     SECTION("Initialize returns device-not-found when DXVK NVAPI hack is enabled") {
+        ::SetEnvironmentVariableA("DXVK_ENABLE_NVAPI", "");
+
         ALLOW_CALL(adapter, GetDesc1(_))
             .SIDE_EFFECT(_1->VendorId = 0x1002)
             .RETURN(S_OK);
@@ -52,6 +54,21 @@ TEST_CASE("Initialize succeeds", "[.sysinfo]") {
         REQUIRE(NvAPI_Unload() == NVAPI_API_NOT_INITIALIZED);
     }
 
+    SECTION("Initialize returns OK when DXVK reports other vendor but DXVK_ENABLE_NVAPI is set") {
+        ::SetEnvironmentVariableA("DXVK_ENABLE_NVAPI", "1");
+
+        ALLOW_CALL(adapter, GetDesc1(_))
+            .SIDE_EFFECT(_1->VendorId = 0x1002)
+            .RETURN(S_OK);
+
+        SetupResourceFactory(std::move(dxgiFactory), std::move(vulkan), std::move(nvml), std::move(lfx));
+
+        REQUIRE(NvAPI_Initialize() == NVAPI_OK);
+        REQUIRE(NvAPI_Unload() == NVAPI_OK);
+
+        ::SetEnvironmentVariableA("DXVK_ENABLE_NVAPI", "");
+    }
+
     SECTION("Initialize returns device-not-found when adapter with non NVIDIA driver ID has been found") {
         ALLOW_CALL(*vulkan, GetPhysicalDeviceProperties2(_, _, _)) // NOLINT(bugprone-use-after-move)
             .SIDE_EFFECT(