STR-889: Make prover CI workflow run only on pull requests

[evgenyzdanovich]

Description

Make prover CI workflow run only on pull requests (thus disabling the run on merge).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature/Enhancement (non-breaking change which adds functionality or enhances an existing one)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor
• New or updated tests
• Dependency Update

Notes to Reviewers

Checklist

• I have performed a self-review of my code.
• I have commented my code where necessary.
• I have updated the documentation if needed.
• My changes do not introduce new warnings.
• I have added tests that prove my changes are effective or that my feature works.
• New and existing tests pass with my changes.

Related Issues

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.35%. Comparing base (78aa39f) to head (d8dbfe0).
Report is 21 commits behind head on main.

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
- Coverage   56.37%   56.35%   -0.03%     
==========================================
  Files         316      316              
  Lines       32754    32754              
==========================================
- Hits        18466    18458       -8     
- Misses      14288    14296       +8     

see 4 files with indirect coverage changes

[github-actions]

Commit: be68b74

SP1 Performance Test Results

program	cycles	success
BTC_BLOCKSPACE	30,357,421	✅
EL_BLOCK	98,789	✅
CL_BLOCK	57,378	✅
L1_BATCH	30,387,324	✅
L2_BATCH	5,473	✅
CHECKPOINT	15,895	✅

[storopoli]

ACK d8dbfe0