linux_xanmod: update script improvements

- Specify nixpkgs in nix-shell for more consistency
- Remove trailing slash in release URLs, which can mess up field slicing

pkgs/os-specific/linux/kernel/update-xanmod.sh

···
 #!/usr/bin/env nix-shell
-#!nix-shell -i bash -p bash nix-prefetch curl jq gawk gnused nixfmt
+#!nix-shell -I nixpkgs=./. -i bash -p bash nix-prefetch curl jq gawk gnused nixfmt
 
 set -euo pipefail
 
···
 RELEASE_URLS=$(echo "$RELEASES" | jq '.[].assets.links.[0].name')
 
 while IFS= read -r url; do
+    # Remove trailing slash
+    url="${url%/}"
+
     # Get variant, version and suffix from url fields:
     #                 8         9       NF
     #                 |         |       |