Add ".external-pins/file.sh" helper so we don't encode our "___" assumption elsewhere

.external-pins/file.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+dir="$(dirname "$BASH_SOURCE")"
+
+for img; do
+	if [[ "$img" != *:* ]]; then
+		echo >&2 "error: '$img' does not contain ':' -- this violates our assumptions! (did you mean '$img:latest' ?)"
+		exit 1
+	fi
+
+	imgFile="$dir/${img/:/___}" # see ".external-pins/list.sh"
+	echo "$imgFile"
+done

.external-pins/update.sh

@@ -18,7 +18,7 @@ for img; do
 
 	digest="$(bashbrew remote arches --json "$img" | jq -r '.desc.digest')"
 
-	imgFile="$dir/${img/:/___}" # see ".external-pins/list.sh"
+	imgFile="$("$dir/file.sh" "$img")"
 	imgDir="$(dirname "$imgFile")"
 	mkdir -p "$imgDir"
 	echo "$digest" | tee "$imgFile"