Build/Test Tools: Remove npx commands in the 6.8 branch

[adimoldovan]

Backport of r63309, already in trunk via #13021, to the 6.8 branch. It replaces the two remaining npx calls in Gruntfile.js with npm exec --no, which fails on a missing package instead of installing it.

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

Testing instructions

1. Run npm ci.
2. Run npm ls update-browserslist-db. It is now a direct devDependency at 1.1.1, and the copy under browserslist is deduped, so the tree gains no package.
3. Run npm exec --no -- update-browserslist-db --version. It prints browserslist-lint 1.1.1 and downloads nothing.
4. Move node_modules/update-browserslist-db aside and run npm exec --no -- update-browserslist-db again. It fails with npx canceled due to missing packages and no YES option instead of fetching the package. Move the directory back.
5. Run npm exec --no -- wp-scripts --help. It runs the installed binary and prints Script name is missing.

Use of AI Tools

AI assistance: Yes — Claude Code (Claude Opus 5) applied r63309 to the 6.8 branch and verified the changeset.

[github-actions]

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[github-actions]

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