ci: add pull-request URL in commit emails

[dkm]

Use github API through gh to get the related pull-request URL and add them in the comment section of the emails.

Example content it creates:

This change was merged into the gccrs repository and is posted here for
upstream visibility and potential drive-by review, as requested by GCC
release managers. 
Each commit email contains a link to its details on github from where you can
find the Pull-Request and associated discussions. 


Commit on github: https://github.com/dkm/gccrs/commit/a4b3fa2b28a59930e9ee8a690845739bf70f3a2f
                                                                           
The commit has been mentionned in the following pull-request(s): 
 - https://github.com/dkm/gccrs/pull/105

 gcc/rust/lang-specs.h | 2 ++
 1 file changed, 2 insertions(+)