ci/update-pinned.sh: import Nixpkgs relative to script

Paths within a --expr expression are relative to the script file,
while relative paths in NIX_PATH are relative to the caller's CWD.

We want the Nixpkgs import to be relative to update-pinned.sh itself,
so that it does not matter where it's executed from.

See https://github.com/NixOS/nixpkgs/issues/425551

(cherry picked from commit 37aac0fb06)

ci/update-pinned.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env nix-shell
-#!nix-shell -i bash -p npins -I nixpkgs=../
+#!nix-shell -i bash -E 'with import ../. {}; mkShell { packages = [ npins ]; }'
 
 set -euo pipefail