tests/env-godebug: simplify initramfs finding

Just get at the initramfs using a glob instead of trying to parse the
BLS. This will work regardless of how the BLS entry is named but assumes
that there is only one BLS entry (which should always be the case for
these tests).

(cherry picked from commit 1841132)

---

We're backporting this because a recent rebuild of 4.15 cosa pulled in a
new ostree which enables `bootloader-naming-2` by default, which changes
the BLS entry name:

ostreedev/ostree#3206
(cherry picked from commit 72bd4b0)

tests/kola/files/env-godebug

@@ -10,9 +10,7 @@ set -xeuo pipefail
 
 . $KOLA_EXT_DATA/commonlib.sh
 
-source /etc/os-release
-ostree_conf="/boot/loader.1/entries/ostree-1-${ID}.conf"
-initramfs=/boot$(grep initrd ${ostree_conf} | sed 's/initrd //g')
+initramfs=$(ls /boot/ostree/*/initramfs-*)
 conf="etc/systemd/system.conf.d/10-default-env-godebug.conf"
 tempd=$(mktemp -d)
 # unpack 10-default-env-godebug.conf from initramfs file