build, tools: drop leading / from r2dir

The script is commented as removing `/home/dist/` part but the `cut`
command is off by one and end up including the `/` character (so that
the resulting string starts with `/`). When this is substituted into
`s3://${cloudflare_bucket}/${r2dir}/${shafile}.asc` we end up with
`//` (one from the template and one from the `r2dir`) which appears
to cause Cloudflare to create an extra top level `/` directory in the
bucket.

PR-URL: #53951
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/release.sh

@@ -158,7 +158,7 @@ sign() {
       # since the promotion script should take care of uploading them.
 
       # Remove /home/dist/ part
-      r2dir=$(echo "$shadir" | cut -c 11-)
+      r2dir=$(echo "$shadir" | cut -c 12-)
 
       # Copy SHASUMS256.txt.asc
       # shellcheck disable=SC2086,SC2029