With nsswitch db_home of /cygdrive/c/%U ssh can't seem to use .ssh folder

[Byron <anondaymia at gmail dot com>]

Every time I use ssh to a machine I get the fingerprint warnings like
it's the first time I've ssh-ed to that machine. I've narrowed it down
to have something to do with my `db_home` being set to `/cygdrive/c/%U`
in `nsswitch.conf`. I have it set to this value because I want my
Cygwin home folder to be the home folder of my computer. Since I'm on
an Active Directory network if I set `db_home` to `windows` then my
home folder is on a network file server rather than my desktop.

I can't see any reason that ssh isn't able to properly use my `.ssh`
folder. Am I missing something, or is this a bug?

Below is sample output of ssh, the permissions of my `.ssh/` folder and
files, and the contents of my `/etc/nsswitch.conf`.

```
$ ssh destination
Could not create directory '/cygdrive/c/bkboulton/.ssh'.
The authenticity of host 'destination (192.168.11.7)' can't be
established. RSA key fingerprint is
SHA256:WKewPf3j9Vljltmsnr/cwZmr0XBy8L5E6Chwp1gWK4g. Are you sure you
want to continue connecting (yes/no)?
```

Permissions of my `/cygdrive/c/bkboulton/.ssh` folder ar `drwx------+`.
Permissions on all file in that folder are `-rw-r--r--`.

```
$ cat /etc/nsswitch.conf
# /etc/nsswitch.conf
#
# passwd:   files db
# group:    files db
db_home:  /cygdrive/c/%U #cygwin desc
db_shell: /usr/bin/zsh
# db_gecos: cygwin desc
```

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