fix(libnpmexec): install npx cache despite dryRun or packageLockOnly

[ljharb]

When npm exec/npx or npm init <initializer> needs a package that is not installed, libnpmexec installs it to the npx cache and then runs its bin. If dryRun or packageLockOnly is set, Arborist skips that install, so the bin is missing and the command fails, even though the user never asked for the exec itself to be a dry run.

This happens without anyone passing those flags to npx directly: lifecycle scripts inherit the parent command's config as npm_config_* environment variables, so a prepublishOnly script that runs npx some-tool fails under npm publish --dry-run, because it sees npm_config_dry_run=true.

#4843 fixed this for packageLockOnly in npm exec only, by overriding it in lib/commands/exec.js. npm init <initializer> calls libnpmexec too, and still had the bug for both flags, and npm exec still had it for dryRun. This moves that override into libnpmexec, where the npx cache's Arborist is created, and extends it to dryRun, so every caller gets it. The existing npm exec packageLockOnly test still passes, and there are new tests in libnpmexec for both flags and in npm exec for dry-run.