[fix](planner) fix show variable display wrong enable_nereids_planner value (#50746)

### What problem does this PR solve?

fix show variable display wrong enable_nereids_planner value, introduced
by #49913

### Release note

None

### Check List (For Author)

- Test <!-- At least one of them must be included. -->
    - [x] Regression test
    - [ ] Unit Test
    - [ ] Manual test (add detailed scripts or steps below)
    - [ ] No need to test or manual test. Explain why:
- [ ] This is a refactor/code format and no logic has been changed.
        - [ ] Previous test can cover this change.
        - [ ] No code files have been changed.
        - [ ] Other reason <!-- Add your reason?  -->

- Behavior changed:
    - [x] No.
    - [ ] Yes. <!-- Explain the behavior change -->

- Does this need documentation?
    - [ ] No.
- [ ] Yes. <!-- Add document PR link here. eg:
https://github.com/apache/doris-website/pull/1214 -->

### Check List (For Reviewer who merge this PR)

- [ ] Confirm the release note
- [ ] Confirm test cases
- [ ] Confirm document
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fe/fe-core/src/main/java/org/apache/doris/qe/StmtExecutor.java

@@ -62,6 +62,7 @@ import org.apache.doris.analysis.SetType;
 import org.apache.doris.analysis.SetVar;
 import org.apache.doris.analysis.SetVar.SetVarType;
 import org.apache.doris.analysis.ShowStmt;
+import org.apache.doris.analysis.ShowVariablesStmt;
 import org.apache.doris.analysis.SqlParser;
 import org.apache.doris.analysis.SqlScanner;
 import org.apache.doris.analysis.StatementBase;
@@ -900,7 +901,8 @@ public class StmtExecutor {
                 throw new TException("This is in a transaction, only insert, commit, rollback is acceptable.");
             }
             SessionVariable sessionVariable = context.getSessionVariable();
-            if (!(parsedStmt instanceof SetStmt) && !(parsedStmt instanceof UnsetVariableStmt)) {
+            if (!(parsedStmt instanceof SetStmt) && !(parsedStmt instanceof UnsetVariableStmt)
+                    && !(parsedStmt instanceof ShowVariablesStmt)) {
                 sessionVariable.disableNereidsPlannerOnce();
             }