verify-action-build: normalize package.json before comparing

[potiuk]

Found while triaging #1100 (reactivecircus/android-emulator-runner), which
fails verify on four checks. Three are tooling gaps; this fixes them.

package.json is rewritten at install time

A vendored node_modules holds the manifest npm rewrote when installing,
while the registry tarball and a fresh npm ci hold the published one.
Comparing bytes reports that difference as tampering. Two separate rewrites:

• Install bookkeeping — npm writes _-prefixed keys that the tarball never
  had. diff_node_modules already filtered these, but by enumeration, and the
  list was missing _args and _location. Now matched by the reserved _
  prefix, since the set has varied across npm versions.
• Shorthand expansion — normalize-package-data turns an author string
  into a person object, bugs into {"url": ...}, and prefixes
  repository.url with git+.

node_modules/tunnel/package.json hits both: it was the only differing file
out of 217, and on its own failed the whole JS build check and the registry
check.

Normalisation is deliberately narrow. Only the fields npm is known to rewrite
are touched; everything else — main, bin, scripts, dependencies — is
still compared strictly, with a regression test asserting a tampered main
still fails.

Two smaller fixes

• @actions/tool-cache's 7zdec.exe is exempted from the in-tree binary
  check. It backs tc.extractZip on Windows and ships inside the first-party
  npm package, so every action vendoring node_modules carries it verbatim
  from the published tarball. Same narrow path-suffix form as the
  gradle-wrapper.jar exemption in verify: exempt gradle-wrapper.jar from in-tree binary check #951. The general fix — crediting any
  binary under a node_modules package the registry check already verified —
  needs the two checks reordered, so I left it for a follow-up.
• The registry mismatch line passed the offending path as the URL and the
  tree root as the text, so it never named the failing file. Diagnosing Pin reactivecircus/android-emulator-runner #1100
  meant deriving the filename from the rebuild diff instead.

Test plan

• uv run pytest utils/tests/ — 320 passed.
• Normaliser checked against the real bytes: committed
  node_modules/tunnel/package.json at a421e438 vs the published
  tunnel-0.0.6.tgz from the registry now compare equal.
• prek run --all-files clean.

Not yet re-run end-to-end against the action itself — that needs a
GITHUB_TOKEN and I exhausted the unauthenticated rate limit. CI on this PR
will cover it.