fix: most-recently-nightly-tag does not assume a 'nightly' remote (#2947)

`script/most-recent-nightly-tag.sh` determines the most recent nightly
release in your current git history.

Previously it was assuming that you had a `nightly` remote, to pull tags
from. Now it just pulls directly from the repository by URL.

script/most-recent-nightly-tag.sh

@@ -1,13 +1,10 @@
 #!/bin/bash
 
-# Name of the remote repository
-remote_name="nightly"
-
 # Prefix for tags to search for
 tag_prefix="nightly-"
 
 # Fetch all tags from the remote repository
-git fetch $remote_name --tags > /dev/null
+git fetch git@github.com:leanprover/lean4-nightly.git --tags > /dev/null
 
 # Get the most recent commit that has a matching tag
 tag_name=$(git tag --merged HEAD --list "${tag_prefix}*" | sort -rV | head -n 1 | sed "s/^$tag_prefix//")