Running process as another user causes permission denied error when setting up database connection.

• asyncpg version: 0.25.0
• PostgreSQL version: 13
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: Both RDS and Docker. Not a PG related issue.
• Python version: 3.8.11 (RedHat)
• Platform: CentOS 7
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: n/a
• Can the issue be reproduced under both asyncio and
  uvloop?: Untested, but shouldn't be relevant.

And I ran into this issue when upgrading from 0.23.0 to 0.25.0, but I believe it was introduced in 0.25.0 based on my reading of the git history.

I am using supervisord to run an app as a different user. When it gets to https://github.com/MagicStack/asyncpg/blob/master/asyncpg/connect_utils.py#L542 when setting up a connection. That resolves the path to /root/.postgresql/postgresql.key and results in an a Permission denied error when it calls .exists() on a file within /root/ as a non root user.

I believe this is because http://supervisord.org/subprocess.html#subprocess-environment doesn't change HOME in the path, and pathlib still sees HOME="/root". So I will be tinkering with unsetting that.

Even if changing the environment fixes my issue, it may be worth adding exception handling for when those paths resolve to a directory the user running asyncpg cannot access.

Start in asyncpg/connect_utils.py around line 542 and reproduce connection setup as a non-root user with HOME pointing to /root. Check how inaccessible PostgreSQL certificate paths are handled when .exists() is called; done means connection setup no longer raises PermissionError for an inaccessible path, with coverage for the reported environment.