Fix OpenBSD Vagrant

This should address #8506. Unfortunately I'm unable to test.

#8506 is likely caused by the Vagrant box having a mirror in its
`etc/installurl`, which does not offer 7.4 packages. There are other
mirrors out there who do, e.g., https://ftp.eu.openbsd.org/pub/OpenBSD/.

Proposed 'fix' is to replace the mirror in `/etc/installurl`.

Vagrantfile

@@ -73,6 +73,7 @@ end
 
 def packages_openbsd
   return <<-EOF
+    echo "https://ftp.eu.openbsd.org/pub/OpenBSD" > /etc/installurl
     pkg_add bash
     chsh -s bash vagrant
     pkg_add xxhash