Build/Test Tools: Remove npx commands in 6.6

[adimoldovan]

Backport of r63309, already in trunk via #13021, to the 6.6 branch. It replaces the two remaining npx calls in Gruntfile.js with npm exec --no, which runs an installed binary instead of downloading and installing one, and adds update-browserslist-db as a direct dev dependency.

Trac ticket: https://core.trac.wordpress.org/ticket/65864

Testing instructions

No workflow runs wp-packages:update or browserslist:update. Both are reached only through grunt sync-gutenberg-packages, a manual release-lead task. CI therefore does not exercise this change, so check it locally.

1. Run npm ci.
2. Run npm exec --no -- update-browserslist-db --version. It prints browserslist-lint 1.0.16 and downloads nothing.
3. Run npm exec --no -- wp-scripts --version. It prints Script name is missing. That message comes from the installed binary, and nothing is downloaded.
4. Run rm -rf node_modules/update-browserslist-db, then run npm exec --no -- update-browserslist-db --version again. It now fails with npx canceled due to missing packages and no YES option instead of fetching the package. Run npm ci to restore the tree.
5. Run npm ls update-browserslist-db. It reports one copy at 1.0.16, both as a direct dev dependency and deduped under browserslist.
6. Run npx --no grunt jshint:grunt. It reports 1 file lint free.

Step 4 is the point of the change. npx installs the missing package and runs its install scripts. npm exec --no refuses.

Use of AI Tools

AI assistance: Yes — Claude Code (Claude Opus 5) applied r63309 to the 6.6 branch and verified the changeset.

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