ci: avoid trying to send slack notification in forks

[tisonkun]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

What's changed and what's your intention?

Although forks can disable this workflow at all, let's try to skip this step because forks doesn't have the secrets: https://github.com/tisonkun/greptimedb/actions/runs/8962181455/job/24610696840

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.59%. Comparing base (73fe075) to head (b3632ab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3857      +/-   ##
==========================================
- Coverage   85.60%   85.59%   -0.01%     
==========================================
  Files         955      955              
  Lines      163262   163262              
==========================================
- Hits       139761   139752       -9     
- Misses      23501    23510       +9     

[v0y4g3r]

LGTM