ci: install release-plz from prebuilt binary

[wavekat-eason]

Problem

Every push to main spends ~12.5 minutes in release-plz-release, of which 8m34s is compiling release-plz from source. This happens even on no-op runs that release nothing.

release-plz/action installs release-plz with cargo-binstall --strategies=crate-meta-data,compile (hardcoded in its composite steps). On the self-hosted wavekat-ci runner, binstall's fetchers exceed their resolution deadline before finding the prebuilt release asset, so the compile strategy wins:

INFO  resolve: Resolving package: 'release-plz@=0.3.160'
WARN  resolve: Timeout reached while checking fetcher invalid url: deadline has elapsed   (47s)
WARN  resolve: Timeout reached while checking fetcher invalid url: deadline has elapsed   (15s)
      Installing release-plz v0.3.160        ← source build, ~600 crates

The same action in the same run on ubuntu-latest (release-plz-pr) resolves the prebuilt in 2.2 seconds. The difference is egress speed from the self-hosted runner — identical tarballs take ~25x longer to fetch there, which is enough to trip binstall's deadline.

Measured from run 30181095552:

Stage	self-hosted	ubuntu-latest
Set up job	65s	5s
binstall resolve	62s, then fails	2.2s, prebuilt
Install release-plz	8m34s (source)	instant
Actual release work	~50s	~50s
Total	12m32s	1m06s

Fix

Install release-plz with taiki-e/install-action, which downloads the prebuilt binary from GitHub Releases directly, then invoke release-plz release with the same arguments the action's composite steps use.

• release-plz@0.3.160 is present in install-action's manifest (currently its latest).
• cargo-semver-checks stays pinned at 0.48 — what the action was already installing — so the only variable is install speed.
• fallback: none is deliberate: without it, a version missing from install-action's manifest silently falls back to cargo-binstall and restores the 10-minute source build with nothing in the log to explain it. Now it fails loudly.
• Git user config is repo-local rather than --global, since the runner is shared.

Expected: ~12.5 min → ~1.5 min.

Verification

release-plz.yml only triggers on push to main, so this can't be exercised by the PR itself — it gets verified on the first push after merge.

Not included

• release-plz-pr still runs on ubuntu-latest using the action (fast there, but ~1 min of billed GH-minutes per push — could move to wavekat-ci now that the install path is fixed).
• The release job still runs on every push to main. At ~1.5 min the no-op case is probably not worth gating.

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