Support BORG_HOSTNAME and BORG_USERNAME env vars, fixes #9651

[ThomasWaldmann]

Adds two environment variables to override the hostname/username borg uses:

• BORG_HOSTNAME — overrides the hostname
• BORG_USERNAME — overrides the username

When set, the override is used both for the value stored in newly created
archives and for the {hostname} / {user} placeholders (so it also
applies to archive names, prune --glob-archives '{hostname}-*', check, etc.).

Use case (from #9651): run borg on host A but impersonate host B — e.g. a
reliably-online desktop performs prune/compact/check on behalf of an
opportunistically-online laptop against a shared, deduplicated repository, and
wants consistent per-hostname archive naming / prune configs.

Fixes #9651.

Implementation

The env vars are read at the point of use (os.environ.get('BORG_HOSTNAME') or hostname),
in the two places that consume these values:

• Archive.save() in src/borg/archive.py (stored archive metadata)
• replace_placeholders() in src/borg/helpers/parseformat.py ({hostname}/{user})

fqdn, hostid (lock stale-removal, see BORG_HOST_ID) and the normal
auto-detection are intentionally left untouched.

Scope note

Per maintainer decision, this is env-vars only — no borg create --hostname/--username
CLI options (unlike the borg2/master change #9402).

Docs / tests

• Documented both vars in the environment variables reference.
• Changelog entry under 1.4.5.
• New test test_create_hostname_username_override verifies both the stored
  metadata and {hostname}-{user} placeholder substitution in the archive name.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.06%. Comparing base (b09bbed) to head (62e3013).
⚠️ Report is 4 commits behind head on 1.4-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9810      +/-   ##
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+ Coverage      82.04%   82.06%   +0.01%     
=============================================
  Files             38       38              
  Lines          11384    11384              
  Branches        1794     1794              
=============================================
+ Hits            9340     9342       +2     
+ Misses          1460     1459       -1     
+ Partials         584      583       -1     

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