When a worktree is reused by actions/checkout and the first time sparse checkout was enabled, we need to ensure that the second time it is only a sparse checkout if explicitly asked for. Otherwise, we need to disable the sparse checkout so that a full checkout is the outcome of this Action.

## Details
* If no `sparse-checkout` parameter is specified, disable it

This should allow users to reuse existing folders when running
`actions/checkout` where a previous run asked for a sparse checkout but
the current run does not ask for a sparse checkout.

This fixes #1475

There are use cases in particular with non-ephemeral (self-hosted) runners where an
existing worktree (that has been initialized as a sparse checkout) is
reused in subsequent CI runs (where `actions/checkout` is run _without_
any `sparse-checkout` parameter).

In these scenarios, we need to make sure that the sparse checkout is
disabled before checking out the files.

### Also includes:

* npm run build
* ci: verify that an existing sparse checkout can be made unsparse
* Added a clarifying comment about test branches.
* `test-proxy` now uses newly-minted `test-ubuntu-git` container image from ghcr.io

---------

Signed-off-by: Johannes Schindelin <[email protected]>
Co-authored-by: John Wesley Walker III <[email protected]>

* Give `test-ubuntu-git` its own `README`
* Refined title and description in `test-ubuntu-git.Dockerfile`
* nit accepted:  Remove double space

---------

Co-authored-by: Ferenc Hammerl <[email protected]>

…1623)

* Warn on attempts to publish test-ubuntu-git from non-main branch.
* Rename build step to clarify that Push is optional.

Bumps [tough-cookie](https://github.com/salesforce/tough-cookie) from 4.0.0 to 4.1.3.
- [Release notes](https://github.com/salesforce/tough-cookie/releases)
- [Changelog](https://github.com/salesforce/tough-cookie/blob/master/CHANGELOG.md)
- [Commits](salesforce/tough-cookie@v4.0.0...v4.1.3)

---
updated-dependencies:
- dependency-name: tough-cookie
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Bumps [@babel/traverse](https://github.com/babel/babel/tree/HEAD/packages/babel-traverse) from 7.20.5 to 7.24.0.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.24.0/packages/babel-traverse)

---
updated-dependencies:
- dependency-name: "@babel/traverse"
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

* Add v4.1.2 and v4.1.1 to CHANGELOG.md
* Bump version in package.json to 4.1.2
* Ran `npm i` to regenerate package-lock.json