Unable to access $SNAP_COMMON for Certificates and Private key path for Encryption settings

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to report a bug and not [ask a question or ask for help](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a)

- [X] I have set up AdGuard Home correctly and [configured clients to use it](https://github.com/AdguardTeam/AdGuardHome/wiki/Clients). (Use the [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) for help with installing and configuring clients.)

### Platform (OS and CPU architecture)

Linux, ARM64

### Installation

Snapcraft

### Setup

On one machine

### AdGuard Home version

0.107.40

### Action

Under Encryption settings
For the pathing of Certificates and Private key, there is no documentation or hint on what is the actual path that will work for snap package.
I tried copying the cert to /var/snap/adguard-home/common but AdguardHome do not seems to find the file over there.
I tried /var/snap/adguard-home/current as well.
Looking through the snap documentation, $SNAP_COMMON would be the current environment variable to use to determine the current path but that does not seems to get evaluated correctly either.

### Expected result

$SNAP_COMMON should be support or documentation to provide a hint on where the root directory that AdguardHome is trying to find the cert and key.

### Actual result

reading key file: open $SNAP_COMMON/domain.key: no such file or directory

### Additional information and/or screenshots

_No response_