util: Remove action runner add-apt-repo git-core/ppa (#856)

We were having some difficulty on a server running this
`apt-apt-repository` command due to suspected firewall issues. On
further inspection is appear to be superfluous as git can be obtained
easily through `apt-get` without adding this repository.

util/github-runners-vagrant/provision_root.sh

@@ -31,7 +31,6 @@ set -eu -o pipefail # -x: is for debugging
 
 apt-get update
 apt-get upgrade -y
-add-apt-repository --yes --update ppa:git-core/ppa
 apt-get install -y \
   software-properties-common \
   bash \