install/strip broken when absolute DESTINATION is used in install() (on Windows)

While trying to figure out #16858 I realized an absolute DESTINATION in install() commands completely breaks the install/strip target:

For a minimal CMakeLists

project (hello)
add_executable(hello helloworld.cpp)
install (TARGETS hello DESTINATION "E:\\bin")

the following output is created in cmake_install.cmake

file(INSTALL DESTINATION "E:/bin" TYPE EXECUTABLE FILES "E:/Temp/Project/build/hello.exe")
  if(EXISTS "$ENV{DESTDIR}/E:/bin/hello.exe" AND
     NOT IS_SYMLINK "$ENV{DESTDIR}/E:/bin/hello.exe")
    if(CMAKE_INSTALL_DO_STRIP)
      execute_process(COMMAND "C:/msys64/mingw64/bin/strip.exe" "$ENV{DESTDIR}/E:/bin/hello.exe")
    endif()
  endif()

Obviously already the call to if(EXISTS ...) is bound to fail due to the illegal $ENV{DESTDIR}/ prefix.

---

Even for relative paths this is prone to failure:

For a minimal CMakeLists (now with relative path)

project (hello)
add_executable(hello helloworld.cpp)
install (TARGETS hello DESTINATION "bin")

the following output is created in cmake_install.cmake

  file(INSTALL DESTINATION "${CMAKE_INSTALL_PREFIX}/bin" TYPE EXECUTABLE FILES "E:/Temp/Project/build/hello.exe")
  if(EXISTS "$ENV{DESTDIR}${CMAKE_INSTALL_PREFIX}/bin/hello.exe" AND
     NOT IS_SYMLINK "$ENV{DESTDIR}${CMAKE_INSTALL_PREFIX}/bin/hello.exe")
    if(CMAKE_INSTALL_DO_STRIP)
      execute_process(COMMAND "C:/msys64/mingw64/bin/strip.exe" "$ENV{DESTDIR}${CMAKE_INSTALL_PREFIX}/bin/hello.exe")
    endif()
  endif()

As soon as CMAKE_INSTALL_PREFIX is set to an absolute path (for example the default C:\Program Files, but also pretty much any path variable CMake has to offer) the code will fail if DESTDIR is defined.

As a matter of fact I believe ${CMAKE_INSTALL_PREFIX} even has to be absolute (unless I'm missing something) as relative paths would be relative to the root of the hard drive. (At least if I use DCMAKE_INSTALL_PREFIX="" in the example the executable ends up in E:\bin\)