Build/Test Tools: Remove npx commands in 6.4

[adimoldovan]

Backport of r63309 to the 6.4 branch, already in trunk via #13021. It replaces the two remaining npx calls with npm exec --no, which runs an installed binary only instead of first downloading and installing a missing package and its dependencies.

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

Testing Instructions

Both Grunt tasks are manual release-lead tools. No workflow runs them, so check them locally.

1. Run npm ci. It completes without errors.
2. Run npm exec --offline --no -- update-browserslist-db --help. It prints the tool's usage text. --offline proves the binary comes from node_modules, not the registry.
3. Run npm exec --offline --no -- wp-scripts --help. The local wp-scripts binary prints Script name is missing.
4. Run npm ls update-browserslist-db. It lists the package as a direct dependency, and again under browserslist as deduped.
5. Run npm exec --no -- a-package-that-is-not-installed. The command fails and installs nothing.

Use of AI Tools

AI assistance: Yes — Claude Code (Claude Opus 5) applied r63309 to 6.4 and verified the changeset.

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