Conda CI test env: depend on pocl>=6
Without this, conda will pick a newer graphviz, which pulls in a newer libxml2, which makes LLVM 15 unavailable, and pushes pocl onto version 1.3.
Without this, conda will pick a newer graphviz, which pulls in a newer libxml2, which makes LLVM 15 unavailable, and pushes pocl onto version 1.3.