Switch to using mirror emacs-mirror/emacs on github

[djgoku]

Maybe switch to github emacs mirror to fix issues like #460

Not sure if this is a welcome PR if not please close it.

% ./update emacs                    
+++ dirname repos/emacs/update
++ cd repos/emacs
++ pwd -P
+ SCRIPTPATH=~/dev/github/djgoku/emacs-overlay/repos/emacs
+ cd ~/dev/github/djgoku/emacs-overlay/repos/emacs
+ update_savannah_branch master
+ branch=master
+ echo emacs master
emacs master
++ curl https://github.com/emacs-mirror/emacs/commits/master.atom
++ xmlstarlet sel -N atom=http://www.w3.org/2005/Atom -t -m /atom:feed/atom:entry -v 'concat(atom:id,'\''/'\'',atom:updated)' -n
++ head -n 1
++ sed 's/^tag:github.com,2008:Grit::Commit\///'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 21351  100 21351    0     0  58934      0 --:--:-- --:--:-- --:--:-- 58980
+ commit_data=92c5c1b786968b6d80f5d27455a40443a3a8f0e8/2025-01-22T23:07:36Z
++ echo 92c5c1b786968b6d80f5d27455a40443a3a8f0e8/2025-01-22T23:07:36Z
++ cut -d / -f 1
+ commit_sha=92c5c1b786968b6d80f5d27455a40443a3a8f0e8
++ echo 92c5c1b786968b6d80f5d27455a40443a3a8f0e8/2025-01-22T23:07:36Z
++ cut -d / -f 2
++ cut -d T -f 1
++ sed s/-//g
+ version_number=20250122.0
++ echo master
++ sed 's/\//_/'
+ output_branch=master
++ nix-prefetch-url --unpack https://github.com/emacs-mirror/emacs/archive/92c5c1b786968b6d80f5d27455a40443a3a8f0e8.tar.gz
path is '/nix/store/5f2ql8ky8ghwyhqqclr9fzb1hmvw9ix5-92c5c1b786968b6d80f5d27455a40443a3a8f0e8.tar.gz'
+ digest=0ahia6jndl20sczzl40ndqs0bj6ddmczqsmq7cghsdh3j53mzign
+ echo '{"type": "github", "owner": "emacs-mirror", "repo": "emacs", "rev": "92c5c1b786968b6d80f5d27455a40443a3a8f0e8", "sha256": "0ahia6jndl20sczzl40ndqs0bj6ddmczqsmq7cghsdh3j53mzign", "version": "20250122.0"}'
[feat/maybe-use-github-emacs-mirror-of-emacs 1abbfcbdc] Updated emacs
 2 files changed, 8 insertions(+), 8 deletions(-)