support/cmake_install.cmake ignores DESTDIR

[realtime-neil]

There's at least one installed path within the install target that is ignoring DESTDIR:

git clean -ffxd \
    && git fetch --all \
    && git pull --rebase \
    && git describe --long --always --dirty \
    && cmake -B build . \
    && make -C build -j$(nproc) \
    && DESTDIR="$(mktemp -dt DESTDIR.XXXXXX)" make -C build install

Removing build/
Already up to date.
b9791c88
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0

...

Traceback (most recent call last):
  File "/home/luser/retdec/support/install-share.py", line 93, in <module>
    main()
  File "/home/luser/retdec/support/install-share.py", line 47, in main
    os.makedirs(support_dir, exist_ok=True)
  File "<frozen os>", line 215, in makedirs
  File "<frozen os>", line 225, in makedirs
PermissionError: [Errno 13] Permission denied: '/usr/local/share/retdec'
CMake Error at support/cmake_install.cmake:77 (message):
  RetDec share directory installation FAILED
Call Stack (most recent call first):
  cmake_install.cmake:71 (include)


make: *** [Makefile:100: install] Error 1