fix: remove WITH_SYSTEM_LLVM env check from conanfile.py

WITH_SYSTEM_LLVM is a CMake cached variable; Conan only needs to know
whether to pull llvm-core, controlled by the WITH_LLVM env var alone.

Author: cphurley82@gmail.com

conanfile.py

@@ -45,7 +45,7 @@ def requirements(self):
         self.requires("jsoncpp/1.9.6")
         self.requires("zlib/1.3.1")
         self.requires("asmjit/cci.20240531")
-        if "WITH_LLVM" in os.environ and "WITH_SYSTEM_LLVM" not in os.environ:
+        if "WITH_LLVM" in os.environ:
             self.requires("llvm-core/19.1.7")
         if os.path.isdir("dbt-rise-plugins"):
             self.requires("lua/5.4.3")