Try using fancy environment modification features.

https://discourse.cmake.org/t/variables-for-cross-platform-code/10115/8?u=dabrahams

cmake/modules/FindSwiftXCTest.cmake

@@ -126,8 +126,8 @@ function(add_swift_xctest test_target testee)
       # If the executable target depends on DLLs their directories need to be injected into the PATH
       # or they won't be found and the target will fail to run, so invoke it through cmake.  Because
       COMMAND
-        ${CMAKE_COMMAND} -E env
-        "PATH=$<JOIN:$<TARGET_RUNTIME_DLL_DIRS:GenerateXCTestMain>;$ENV{PATH},;>"
+        ${CMAKE_COMMAND} -E env --modify
+        "$<LIST:TRANSFORM,$<TARGET_RUNTIME_DLL_DIRS>,PREPEND,PATH=>"
         --
         $<TARGET_FILE:GenerateXCTestMain> -o ${test_main} ${sources}
       DEPENDS ${sources} GenerateXCTestMain
@@ -150,7 +150,7 @@ function(add_swift_xctest test_target testee)
     # (and interpreted by CMake). [It] is then used to populate CTestTestfile.cmake, which is later
     # read by CTest to setup your test environment.”
     set_tests_properties(${test_target}
-      PROPERTIES ENVIRONMENT "PATH=$<JOIN:$<TARGET_RUNTIME_DLL_DIRS:${test_target}>;${path},\\;>")
+      PROPERTIES ENVIRONMENT_MODIFICATION "$<LIST:TRANSFORM,$<TARGET_RUNTIME_DLL_DIRS>,PREPEND,PATH=>")
 
   endif()