[Bug] "YN0028: The lockfile would have been created by this install, which is explicitly forbidden", but `yarn.lock` exists and is not modified when running `yarn install` locally

• I’d be willing to implement a fix

Describe the bug

When I run yarn install and then git status I can see that there are no changes since the last commit, i. e. yarn.lock does not get modified by running yarn install.

Yet, when I push the commit to CI that has this GitHub Actions step:

      - name: Install dependencies
        run: yarn install

…it fails with:

2021-05-28T13:48:22.9912155Z ➤ YN0028: The lockfile would have been created by this install, which is explicitly forbidden.
2021-05-28T13:48:22.9913523Z ➤ YN0000: Failed with errors in 0s 12ms
2021-05-28T13:48:22.9918681Z ##[error]Process completed with exit code 1.

It’s weird that the error message is saying “created”. I was not able to google up any mentions of this specific error message. All mentions of YN0028 say “modified”, not “created”.

I believe this is a bug since doing yarn install locally does not modify yarn.lock, which exists and is checked into the repo.

To Reproduce

I do not have a minimal reproduction, but here’s a failing CI job: https://github.com/kaliber5/ember-bdd/actions/runs/885833497

It references a commit of a public open source project on GitHub.

I was able to work around the prpblem by setting the YARN_ENABLE_IMMUTABLE_INSTALLS env var to false. But it does not explain why Yarn is not able to use the lockfile.

Environment if relevant (please complete the following information):

• OS: Ubuntu 20.04
• Node version: 14
• Yarn version: 3.0.0-rc.2

Additional context

The project is a Yarn Workspaces monorepo.