GitHub actions that install dependencies will now also cache those
dependencies using the standard strategy for Yarn (which is to hash the
lockfile).
This matches the module template (see https://github.com/MetaMask/metamask-module-template/pull/145
for details).
This should have no functional impact except that this action will run
faster when dependencies are unchanged.
The fitness function workflow now uses an immutable install, ensuring
that the dependencies installed are known and tracked in the lockfile.
This makes it easier to audit exactly which dependencies were used for
each run.
The `nvmrc` file is now referenced in our GitHub Actions workflows,
rather than hard-coding the expected Node.js version. This will make
future Node.js version changes easier to manage.
