[oni-core] Add command to copy the guix build hash

This is convenient when I'm writing Guix packages.

oni-core.el

@@ -527,5 +527,17 @@ which normally have their errors suppressed."
 (when (treesit-available-p)
   (add-to-list 'interpreter-mode-alist '("bash" . bash-ts-mode)))
 
+(defun oni-core-copy-guix-build-hash ()
+  "Try and find the last actual hash in the compilation buffer and insert it."
+  (interactive)
+  (insert
+   (with-current-buffer compilation-last-buffer
+     (save-excursion
+       (save-match-data
+         (goto-char (point-max))
+         (re-search-backward (rx bol (zero-or-more whitespace) "actual hash:" (one-or-more whitespace)))
+         (goto-char (match-end 0))
+         (buffer-substring-no-properties (point) (line-end-position)))))))
+
 (provide 'oni-core)
 ;;; oni-core.el ends here