Change check for zsh instead of macOS

Author: daveisfera

scripts/activate.sh

@@ -6,7 +6,7 @@ if [[ "$0" == "${BASH_SOURCE[0]}" ]]; then
     exit 1
 fi
 
-if [[ "$(uname)" == "Darwin" ]]; then
+if [[ -n "$ZSH_VERSION" ]]; then
     export PYAV_ROOT="$(realpath -- "$(dirname -- "$(readlink -f -- "$0")")/..")"
 else
     export PYAV_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.."; pwd)"