Specify that only amd64 arch is available on this apt repo

The download.newrelic.com apt repo doesn't provide i386 packages, leading to errors when apt tries to update its package index, even on amd64 machines.

By specifying the arch parameter, we're telling Apt that only amd64 packages are available here.

We also add a trailing slash to the URL, because the NR repo server can list empty dirs otherwise.

manifests/repo/infra.pp

@@ -29,7 +29,7 @@
       $require = Apt::Source['newrelic-infra']
       ensure_packages('apt-transport-https')
       ::apt::source { 'newrelic-infra':
-        location => 'https://download.newrelic.com/infrastructure_agent/linux/apt',
+        location => '[arch=amd64] https://download.newrelic.com/infrastructure_agent/linux/apt/',
         repos    => 'main',
         key      => {
           id  => 'A758B3FBCD43BE8D123A3476BB29EE038ECCE87C',