delta backup does not work without $PGDATA in the command line

[IgorOhrimenko]

Database name

postgresql

Issue description

Describe your problem

delta backup does not work without $PGDATA in the command line.
"WALG_DELTA_MAX_STEPS": 7 is set in the .walg.json but when run wal-g backup-push get message:
Features like delta backup are disabled, there might be a performance impact.
If run wal-g backup-push $PGDATA with the same config .walg.json get message Delta backup enabled

Please provide steps to reproduce

WALG_DELTA_MAX_STEPS=1 wal-g backup-push -> Features like delta backup are disabled, there might be a performance impact.
WALG_DELTA_MAX_STEPS=1 wal-g backup-push /var/lib/postgresql/14/cluster-psql-test -> Delta backup enabled

Please add config and wal-g stdout/stderr logs for debug purpose

.walg.json
{
    "WALE_S3_PREFIX": "s3://backup_postgresql_wal-g/cluster-psql-test/14",
    "AWS_ACCESS_KEY_ID": "AWS_ACCESS_KEY_ID",
    "AWS_ENDPOINT": "https://s3",
    "AWS_S3_FORCE_PATH_STYLE": "true",
    "AWS_SECRET_ACCESS_KEY": "pass",
    "AWS_REGION": "reg",
    "PGDATA": "/var/lib/postgresql/14/cluster-psql-test",
    "PGHOST": "/var/run/postgresql",
    "PGPORT": "5432",
    "WALG_DELTA_MAX_STEPS": "7",
    "WALG_PREFETCH_DIR": "/var/lib/postgresql/",
    "WALG_TAR_SIZE_THRESHOLD": "4294967296",
    "WALG_UPLOAD_CONCURRENCY": "1",
    "WALG_COMPRESSION_METHOD": "brotli"
}

This issue has some relatives #1385 and #1248

If you can, provide logs

WALG_DELTA_MAX_STEPS=1 wal-g backup-push
INFO: 2023/10/25 17:02:01.587486 Running remote backup through Postgres connection.
INFO: 2023/10/25 17:02:01.587555 Features like delta backup are disabled, there might be a performance impact.
INFO: 2023/10/25 17:02:01.587558 To run with local backup functionalities, supply [db_directory].
INFO: 2023/10/25 17:02:01.615639 Starting remote backup
INFO: 2023/10/25 17:02:01.721266 Streaming remote backup
INFO: 2023/10/25 17:02:01.732651 Adding data directory
INFO: 2023/10/25 17:02:24.301164 Finishing backup
INFO: 2023/10/25 17:02:24.301199 If wal-g hangs during this step, please Postgres log file for details.
INFO: 2023/10/25 17:02:24.304672 Updating metadata
INFO: 2023/10/25 17:02:24.304711 Uploading metadata
INFO: 2023/10/25 17:02:24.495913 Wrote backup with name base_0000002900000280000000A1

WALG_DELTA_MAX_STEPS=1 wal-g backup-push /var/lib/postgresql/11/cluster-psql-test
INFO: 2023/10/25 17:04:45.001627 LATEST backup is: 'base_0000002900000280000000A1'
INFO: 2023/10/25 17:04:45.055755 Delta backup from base_0000002900000280000000A1 with LSN 280/A1000028.
INFO: 2023/10/25 17:04:45.122151 Calling pg_start_backup()
INFO: 2023/10/25 17:04:45.478021 Delta backup enabled
INFO: 2023/10/25 17:04:45.478051 Starting a new tar bundle
INFO: 2023/10/25 17:04:45.478084 Walking ...
INFO: 2023/10/25 17:04:45.479887 Starting part 1 ...
INFO: 2023/10/25 17:05:00.094650 Packing ...
INFO: 2023/10/25 17:05:00.096655 Finished writing part 1.
INFO: 2023/10/25 17:05:01.702814 Starting part 2 ...
INFO: 2023/10/25 17:05:01.774338 /global/pg_control
INFO: 2023/10/25 17:05:01.812535 Finished writing part 2.
INFO: 2023/10/25 17:05:01.812978 Calling pg_stop_backup()
INFO: 2023/10/25 17:05:05.054343 Starting part 3 ...
INFO: 2023/10/25 17:05:05.054430 backup_label
INFO: 2023/10/25 17:05:05.054439 tablespace_map
INFO: 2023/10/25 17:05:05.057440 Finished writing part 3.
INFO: 2023/10/25 17:05:05.368358 Wrote backup with name base_0000002900000280000000A3_D_0000002900000280000000A1

[x4m]

Thanks for raising the issue!
I think we should somehow prevent user from doing remote backup when we are running locally and have access to FS...