ExternalProject: Fix regression in passing list to CMAKE_CACHE_ARGS

Fix passing a list to the CMAKE_CACHE_ARGS and CMAKE_CACHE_DEFAULT_ARGS
options of ExternalProject_Add.

Following commit v3.7.0-rc1~273^2~1 (prefer list(APPEND) over
string(APPEND) where appropriate, 2016-08-08), the semicolon list
separator after the first list element was missing in the generated
cache.
2 jobs for st2-exclude-patterns-variable
Status Name Job ID Coverage
  External
passed kwrobot-commit-check

passed kwrobot-commit-check