renovate: Improve PR creation reliability (#47502)

Set pnpm's `allowUnusedPatches` to avoid erroring out for cases like
Automattic/jetpack#47472 (comment).
Too bad pnpm 11 is removing `ignorePatchFailures`, that'd be useful for
the PR creation too.

Also, try creating the change entries even if `pnpm install` fails, as
the change entry creation doesn't depend on pnpm. The later
`tools/check-intra-monorepo-deps.sh` run will fail, but getting change
entries created is still an improvement.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/22844210192

Upstream-Ref: Automattic/jetpack@e211e42

Author: sdnunca