[docker] Fix LLVM_EXTERNAL_PROJECTS cmake variable value

Summary:
LLVM_ENABLE_PROJECTS expects a semicolon separated project list.

Fixes PR38158.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49712

llvm-svn: 337842