Build/Test Tools: Remove npx commands in 6.7

[adimoldovan]

Backport of r63309, already in trunk via #13021, to the 6.7 branch. It replaces the two remaining npx calls with npm exec --no, which runs an installed binary and fails when the package is missing, and declares update-browserslist-db as a direct devDependency. That dependency is pinned to 1.1.1, the version this branch already resolves, rather than trunk's 1.3.1, so nothing is upgraded.

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

Testing instructions

1. Run npm ci.
2. Run npm exec --no -- wp-scripts. It prints Script name is missing., the local binary's own output, and downloads nothing.
3. Run npm exec --no -- update-browserslist-db --version. It prints browserslist-lint 1.1.1.
4. Delete node_modules/update-browserslist-db, then run npm exec --no -- update-browserslist-db. It fails with npx canceled due to missing packages and no YES option instead of fetching the package. Restore the tree with npm ci.
5. Run npm exec --no -- grunt --help. Both wp-packages:update and browserslist:update are listed.
6. Run grep -rn npx Gruntfile.js .github/ package.json. It returns nothing.

Use of AI Tools

AI assistance: Yes — Claude Code (Claude Opus 5) applied r63309 to the 6.7 branch and verified the changeset.

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