fix: fix nix flake sed command (coder#13243)

Author: sreya

scripts/update-flake.sh

@@ -13,4 +13,4 @@ echo "Calculating SRI hash..."
 HASH=$(go run tailscale.com/cmd/nardump --sri "$OUT/pkg/mod/cache/download")
 sudo rm -rf "$OUT"
 
-sed -i "s/\(vendorHash = \"\)[^\"]*/\1${HASH}/" ./flake.nix
+sed -i "s#\(vendorHash = \"\)[^\"]*#\1${HASH}#" ./flake.nix