cmake: Fix USER_CACHE_DIR path generation

When the optional env_suffix_${env_var} was not set, USER_CACHE_DIR was
slightly malformed and had double slashes,
e.g. /home/user/.cache//zephyr.

To fix it string(JOIN ...) is used, which only sets slashes when
necessary.

Signed-off-by: Manoel Brunnen <mb@lee-brunnen.de>