ci: only run benchmark in pr event

[KKould]

when the pr is merged, the benchmark action will make an error when commenting because the pr cannot be found.

[github-actions[bot]]

Read:

+--------------------------------------------+----------+---------+
|                                            | tonbo    | rocksdb |
+=================================================================+
| random range reads                         | 124048ms | 36213ms |
|--------------------------------------------+----------+---------|
| random range reads                         | 121795ms | 36102ms |
|--------------------------------------------+----------+---------|
| random range projection reads              | 51548ms  | 36340ms |
|--------------------------------------------+----------+---------|
| random range projection reads              | 51609ms  | 36373ms |
|--------------------------------------------+----------+---------|
| random range projection reads (4 threads)  | 39448ms  | 36402ms |
|--------------------------------------------+----------+---------|
| random range projection reads (8 threads)  | 36715ms  | 36372ms |
|--------------------------------------------+----------+---------|
| random range projection reads (16 threads) | 37098ms  | 35017ms |
|--------------------------------------------+----------+---------|
| random range projection reads (32 threads) | 31582ms  | 34814ms |
+--------------------------------------------+----------+---------+

[github-actions[bot]]

Write:

+-------------------------------+--------+---------+
|                               | tonbo  | rocksdb |
+==================================================+
| bulk load                     | 6484ms | 9648ms  |
|-------------------------------+--------+---------|
| individual writes             | 8172ms | 10852ms |
|-------------------------------+--------+---------|
| individual writes (4 threads) | 9403ms | 10906ms |
|-------------------------------+--------+---------|
| individual writes (8 threads) | 8590ms | 10929ms |
|-------------------------------+--------+---------|
| batch writes                  | 8632ms | 7216ms  |
|-------------------------------+--------+---------|
| batch writes (4 threads)      | 7756ms | 7234ms  |
|-------------------------------+--------+---------|
| batch writes (8 threads)      | 8700ms | 7255ms  |
|-------------------------------+--------+---------|
| removals                      | 3150ms | 3876ms  |
+-------------------------------+--------+---------+