/var/snap/microk8s/common/ 以下に置かれているようだ。
$ sudo find / -name wp-login.php 2>/dev/null
/var/snap/microk8s/common/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/179/fs/opt/bitnami/wordpress/wp-login.php
/var/snap/microk8s/common/var/lib/containerd/io.containerd.snapshotter.v1.overlayfs/snapshots/170/fs/opt/bitnami/wordpress/wp-login.php
/var/snap/microk8s/common/run/containerd/io.containerd.runtime.v2.task/k8s.io/6f704f341ec11315ce29fc36f1a4d4c4e7c88b55d460fa0249c7e268b20be2ed/rootfs/opt/bitnami/wordpress/wp-login.php