ci: fix a bug that open nginx.pid failed

[xuruidong]

Description

There are some failed "CLI Test (master)" workflow runs, such as https://github.com/apache/apisix/actions/runs/5886312117/job/15963999013 .

We can get the following error message in the "Linux Script" job:

Error: 2023/08/17 03:14:30 [error] 32685#32685: open() "/usr/local/apisix/logs/nginx.pid" failed (2: No such file or directory)
Error: Process completed with exit code 1.

The problem code is in ci/linux_apisix_master_luarocks_runner.sh

sudo PATH=$PATH apisix init
    sudo PATH=$PATH apisix start          # step 1
    sudo PATH=$PATH apisix quit           # step 2
    sudo PATH=$PATH apisix start          # step 3
    sudo PATH=$PATH apisix stop

After run apisix quit, if the process does not exit totaly before apisix start (step 3), the apisix start will not create a new process. Then the process created in "step 1" exit and nginx.pid is removed, so apisix stop will cause an error "open nginx.pid failed"
So it is necessary to wait for the process to exit after apisix quit.

Fixes # (issue)

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)