Fix mistake added to main for Valgrind

This reverts commit f43164c.

Author: mcbarton

.github/workflows/Ubuntu.yml

@@ -403,7 +403,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install git g++ debhelper devscripts gnupg python3 doxygen graphviz python3-sphinx
         sudo apt-get install -y libc6-dbg
-        sudo snap install valgrind
+        sudo snap install valgrind --classic
         sudo apt autoremove
         sudo apt clean
         # Install libraries used by the cppyy test suite