[ISSUE-44] Get commit SHA from pull_request.merge_commit_sha

[lampajr]

Fixes #44

As per linked issue, this PR aims to update the way in which the commit SHA to be cherry picked is retrieved.

Instead of getting it from process.env.GITHUB_SHA, it retrieves that SHA from contex.payload.pull_request.merge_commit_sha, whose value change depending on the pull request status [1]

In this way it is possible to cherry-pick pull requests that have merged some time ago, even if new commits have been added on top of the original PR's target branch. This was not possible if using pull_request_target workflow.

[1] https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#get-a-pull-request

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[lampajr]

@carloscastrojumo here it is, I tried an example here where I exeuted the following actions:

1. Merge this first PR Update file1.txt lampajr/backporting-example#41 without backporting it
2. Merge and backport this PR Update file2.txt lampajr/backporting-example#42
3. Add the labels on the first PR in order to backport it, this correctly generated [cherry-pick] Update file1.txt lampajr/backporting-example#46 and [cherry-pick] Update file1.txt lampajr/backporting-example#45 even if on the main branch a new commit was added by the second PR.

NOTE: the backport is triggered using labels, see the workflow definition for more details.

[carloscastrojumo]

Looks awesome @lampajr. Thank you for showing the working example btw.