fix(release): run the deduper from a path that survives the checkout

[derrickmehaffy]

#210's step failed on its first real run — the wiring I flagged as untested.

Error: Cannot find module '/home/runner/work/plugin-rest-cache/plugin-rest-cache/scripts/dedupe-changelog.mjs'

It checks out the release branch, and that branch was cut from 3498f3a —
before the script existed. So it checked out a tree without its own script
in it.

Fix: copy the script to $RUNNER_TEMP before touching the working tree.

A second bug, found by rehearsing properly

Rather than ship untested wiring twice, I replayed the step verbatim against the
real release-please--branches--main. That surfaced a failure the first run
never got far enough to hit:

error: Your local changes to the following files would be overwritten by checkout:
	CHANGELOG.md
Aborting

git checkout - aborts while CHANGELOG.md is dirty. It was only switching
back out of tidiness — step 2 runs entirely through the API with the script
already copied out — so it no longer switches back.

Verified

The step, run verbatim, against the actual release branch:

removed 11 duplicate entries:
  - add a cache statistics endpoint for the admin dashboard
  - add an opt-in content api purge endpoint
  ... (9 more)
committed OK: 85ec3c5 chore: drop duplicate changelog entries
step completed, exit=0

Note

The failure was contained, as designed: it runs after release-please, so #209
was still updated correctly — only the deduplication was skipped. #209 currently
has 73 changelog entries; merging this drops it to 62.

Touches a workflow, so it needs a maintainer to merge.

🤖 Generated with Claude Code