[workflows] Split pr-code-format into two parts to make it more secure (#78216)

Actions triggered by pull_request_target events have access to all
repository secrets, so it is unsafe to use them when executing untrusted
code. The pr-code-format workflow does not execute any untrusted code,
but it passes untrused input into clang-format. An attacker could use
this to exploit a flaw in clang-format and potentially gain access to
the repository secrets.

By splitting the workflow, we can use the pull_request target which is
more secure and isolate the issue write permissions in a separate job.
The pull_request target also makes it easier to test changes to the
code-format-helepr.py script, because the version of the script from the
pull request will be used rather than the version of the script from
main.

Fixes #77142