Before this patch RUN_TESTS condition in Dockerfile.staticbuild was
ignored and always was true.

However adding of brackets solves only part of problem. If RUN_TESTS is
empty `sh -c` returns 1 and build fails.

However if we run tests we should fail build if tests are not passed.
Ternary logic was rewritten to fair if-else.

This patch fixes it and allows build tarantool statically without
running tests.

@Totktonada: Fixed .gitlab.mk to pass RUN_TESTS environment variable to
docker build arguments.

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>