Unclear defaults for `plugin-dir`

[dveeden]

Bug Report

1. Minimal reproduce step (Required)

$ ./bin/tidb-server -help 2>&1 | grep -A1 plugin-dir
  -plugin-dir string
    	the folder that hold plugin (default "/data/deploy/plugin")

5.7.25-TiDB-v5.3.0-alpha-121-gc10331066 127.0.0.1:4000   test  SQL  show session variables like 'plugin_dir';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| plugin_dir    |       |
+---------------+-------+
1 row in set, 1 warning (0.0016 sec)
Warning (code 1287): 'tidb_slow_log_masking' is deprecated and will be removed in a future release. Please use tidb_redact_log instead

./bin/tidb-server -plugin-load foobar-1
...
[2021/09/16 08:49:26.547 +02:00] [FATAL] [terror.go:276] ["unexpected error"] [error="plugin.Open(\"foobar-1.so\"): realpath failed"] ...

./bin/tidb-server -plugin-load foobar-1 -plugin-dir /data/deploy/plugin
...
[2021/09/16 08:50:19.163 +02:00] [FATAL] [terror.go:276] ["unexpected error"] [error="plugin.Open(\"/data/deploy/plugin/foobar-1.so\"): realpath failed"] ...

2. What did you expect to see? (Required)

Consistently see either /data/deploy/plugin or "" as default.

3. What did you see instead (Required)

Different places show different defaults

4. What is your TiDB version? (Required)

$ ./bin/tidb-server -V
Release Version: v5.3.0-alpha-121-gc10331066
Edition: Community
Git Commit Hash: c103310663fabed66259ce7c6e1e98cb3935d909
Git Branch: master
UTC Build Time: 2021-09-16 05:51:00
GoVersion: go1.16.6
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.