server: set system_time_zone from systemTZ (#13934)

[bb7133]

Cherry-pick #13934 without conflict.

---

What problem does this PR solve?

This PR tries to close #13894, by setting system variable system_time_zone from system_tz variable in mysql.tidb table.

What is changed and how it works?

A setSystemTimeZoneVariable() is added to server.NewServer(), which get the value of systemTZ from exported function GetSystemTZ()

Please notice that this PR doesn't change any other behavior: all time-related calculations are not affected.

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)

export TZ="Europe/London"
# Before the initial bootstrap of TiDB Server)

./bin/tidb-server
# TiDB bootstrapped and infer `systemTZ` from TZ environment variable.

tidb> select variable_value from mysql.tidb where variable_name = "system_tz"; 
+----------------+
| VARIABLE_VALUE |
+----------------+
| Europe/London  |
+----------------+
1 row in set (0.00 sec)

tidb> select @@system_time_zone;                                                  
+--------------------+
| @@system_time_zone |
+--------------------+
| Europe/London      |
+--------------------+
1 row in set (0.00 sec)

The scripts described in #13894 is also used for manual tests.

Code changes

• Has exported function/method change

Side effects

• NA

Related changes

• Need to update the documentation

Release note

• Fix the issue that system_time_zone is hard-coded as CST by setting this variable from system_tz in mysql.tidb table.

[zz-jason]

LGTM

[AilinKid]

LGTM

[sre-bot]

Your auto merge job has been accepted, waiting for 13951

[sre-bot]

/run-all-tests

[sre-bot]

@bb7133 merge failed.

[jackysp]

/merge

[sre-bot]

/run-all-tests