manifests: consistently use set -xeuo pipefail in postprocess scripts

The `set -euo pipefail` shouldn't be controversial.

The `set -x` bit is more debatable because it makes
the output more verbose but I find it helpful right
now as I comb through all sorts of details as part of
https://github.com/coreos/fedora-coreos-tracker/issues/1861.

If we care about less verbose output, I'd prefer we keep `-x` but
switched to having rpm-ostree buffer postprocess output and only dump it
if it fails.

minimal/initramfs.yaml

@@ -2,6 +2,7 @@
 postprocess:
   - |
     #!/usr/bin/env bash
+    set -xeuo pipefail
     mkdir -p /usr/lib/dracut/dracut.conf.d
     cat > /usr/lib/dracut/dracut.conf.d/20-bootc-base.conf << 'EOF'
     # We want a generic image; hostonly makes no sense as part of a server side build