maint: update-guix-package: Prevent accidentally breaking guix pull.

Fixes <https://issues.guix.gnu.org/43893>.

This changes the 'update-guix-package' tool so that it:

1. Always uses a clean checkout to compute the hash of the updated 'guix'
package.
2. Ensures the commit used in the updated 'guix' package definition has already
been pushed upstream.

* build-aux/update-guix-package.scm (%savannah-guix-git-repo-push-url): New
variable.
(with-input-pipe-to-string, with-temporary-git-worktree): New syntaxes.
(find-origin-remote, git-add-worktree): New procedures.
(commit-already-pushed?): New predicate.
(main): Check the commit used has already been pushed upstream and compute the
hash from a clean checkout.
* doc/contributing.texi (Updating the Guix Package): Document it.
* .dir-locals.el (scheme-mode): Fix indentation of with-temporary-git-worktree.