ci: bench: fix Resource not accessible by integration on PR event

[phymbert]

Context

Facing Resource not accessible by integration on Benchmark workflow on PR created from forks, example:

• https://github.com/ggerganov/llama.cpp/actions/runs/8478465820/job/23230834051

This is expected according to the github documentation, the GITHUB_TOKEN has read permission on PR from fork.

Change

Switch trigger event from pull_request to pull_request_target.

Explained on github documentation:

For workflows that are triggered by the pull_request_target event, the GITHUB_TOKEN is granted read/write repository permission

As we need write permission for commit status and and PR comment.

[phymbert]

@ggerganov Georgi, let's try this approach, probably need to change some variables inside the workflow later on. But need to be merged first