fix: use RESTIC_HOST for restic snapshots

[w3lld1]

Summary

• I switched the backup script to Restic's native RESTIC_HOST environment variable and removed redundant --host arguments.
• I kept RESTIC_HOSTNAME as a backward-compatible fallback when RESTIC_HOST is not set.
• I updated the Restic integration test to verify that the legacy variable still selects the snapshot host, and documented the preferred variable.

Validation

• bash -n scripts/opt/backup-loop.sh tests/test.simple.restic.sh
• ad-hoc one-shot mock-Restic harness covering native-variable precedence, legacy fallback, environment propagation, and absence of --host
• git diff --check origin/master...HEAD

I deferred the Docker image build and full Restic integration test to CI because the repository workflow performs a full container build before running it.

Fixes #301

[itzg]

Thanks!