ci/update-pinned-nixpkgs.sh: Allow setting the rev

This script only needs to make sure that the revision has the relevant
packages built, it doesn't necessarily need to be a channel version.
This commit makes it possible to set the revision explicitly when
calling the update script.

ci/update-pinned-nixpkgs.sh

@@ -10,7 +10,8 @@ repo=https://github.com/nixos/nixpkgs
 branch=nixpkgs-unstable
 file=$SCRIPT_DIR/pinned-nixpkgs.json
 
-rev=$(git ls-remote "$repo" refs/heads/"$branch" | cut -f1)
+defaultRev=$(git ls-remote "$repo" refs/heads/"$branch" | cut -f1)
+rev=${1:-$defaultRev}
 sha256=$(nix-prefetch-url --unpack "$repo/archive/$rev.tar.gz" --name source)
 
 jq -n --arg rev "$rev" --arg sha256 "$sha256" '$ARGS.named' | tee /dev/stderr > $file