ubuntu-core-initramfs fails in oracular: /etc needs to be created before ldconfig is run

Hi,

Installing kernels (with u-c-i installed) on oracular fails as ldconfig fails to run:

```
Processing triggers for linux-image-unsigned-6.10.0-20-generic (6.10.0-20.20) ...
/etc/kernel/postinst.d/ubuntu-core-initramfs:
/sbin/ldconfig.real: Can't create temporary cache file /etc/ld.so.cache~: No such file or directory
Traceback (most recent call last):
File "/usr/bin/ubuntu-core-initramfs", line 875, in
main()
File "/usr/bin/ubuntu-core-initramfs", line 871, in main
globals()[args.subcmd.replace("-", "_")](parser, args)
File "/usr/bin/ubuntu-core-initramfs", line 650, in create_initrd
install_misc(main, rootfs)
File "/usr/bin/ubuntu-core-initramfs", line 482, in install_misc
check_call(["ldconfig", "-r", dest_dir])
File "/usr/lib/python3.12/subprocess.py", line 413, in check_call
raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['ldconfig', '-r', '/tmp/tmpg_hc5zl_.ubuntu-core-initramfs/main']' returned non-zero exit status 1.
run-parts: /etc/kernel/postinst.d/ubuntu-core-initramfs exited with return code 1
```

creating /etc by adding it to the files installed by install_misc makes sure it exists before ldconfig, so it seems as if it used to be included there indirectly by something else.

This is easy to reproduce on a chroot as well.

Start with the ubuntu-core-initramfs script, especially install_misc and the ldconfig call shown in the traceback. Reproduce the failure in an Oracular chroot, then verify that installing a kernel completes successfully and ldconfig can create its cache under the generated initramfs root.

Written by the indexing model from the issue text.