Fix nix installation

Author: arctan95

.devcontainer/first-run-setup.sh

@@ -98,7 +98,7 @@ fi
 
 echo "Installing Nix..."
 # Check if Nix is already installed
-if [ -e "/nix" ]; then
+if command -v nix >/dev/null 2>&1; then
   echo "Nix already installed. Skipping..."
 else
   # Install Nix
@@ -186,4 +186,4 @@ fi
 echo "Installing asdf plugins and packages..."
 # Install asdf plugins and packages from .tool-versions file
 ln -sf "$HOME/.codespaces/.tool-versions" "$HOME/.tool-versions"
-cut -d ' ' -f1 "$HOME/.tool-versions" | xargs -I {} sh -c 'echo "Installing {}..."; asdf plugin add {} && asdf install {}'
+cut -d ' ' -f1 "$HOME/.tool-versions" | xargs -I {} sh -c 'echo "Installing {}..."; asdf plugin add {} && asdf install {}'