Always checkout JavaScript environment from main

Workflows run from the workflow file on main, even when they
run on branches.  But just running "npm i" or a command like "npx"
that also installs packages uses the package.json file from
the branch.

Rather than attempt to keep the branch package files up-to-date
(which would _quadruple_ the number of update PRs, adding three
branch update PRs for every dependabot PR), let's just checkout
the package.json from main whenver we run.  The other workflows
only run on main - these two are the ones that run on branches.