bugfix(ci): avoid E2BIG in pr_slow_ci_suggestion (#45983)
ci: avoid E2BIG in pr_slow_ci_suggestion by refetching PR files in-script
The "Write pr_files file" step has been bouncing between two failure modes:
* Heredoc with ${{ }} interpolation — template-injection vulnerable.
* `printf '%s\n' "$PR_FILES"` with PR_FILES as an env var — trips
`E2BIG` ("Argument list too long") on PRs with many/large patches,
because a single env value past ~128KB exceeds MAX_ARG_STRLEN on
the execve into bash.
Replace both with a github-script step that calls `pulls.listFiles`
itself and writes the JSON via `fs.writeFileSync`. The payload stays in
Node heap and never crosses an argv/envp boundary, and no PR-controlled
content is ever expanded into a script source.
