dotnet: allow update.sh to be run from root

pkgs/development/compilers/dotnet/update.sh

@@ -1,9 +1,11 @@
 #!/usr/bin/env nix-shell
-#!nix-shell -I nixpkgs=../../../../. -i bash -p curl jq nix gnused
+#!nix-shell -I nixpkgs=./. -i bash -p curl jq nix gnused
 # shellcheck shell=bash
 
 set -Eeuo pipefail
 
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
 release () {
   local content="$1"
   local version="$2"