[content-service] Do not stash untracked files

[aledbf]

Description

Avoiding issues with files included in the prebuild that could be owned by a different issue (permission issues).

Related Issue(s)

Fixes #10953

logs: https://cloudlogging.app.goo.gl/YheW1oLHcTZhkwNf7

How to test

• Open a workspace using https://github.com/aledbf/supervisor-bug
• Check the directory letsencrypt exists and the file acme.json is owned by root

Release Notes

NONE 

Werft options:

• /werft with-preview

[werft-gitpod-dev-com]

started the job as gitpod-build-aledbf-stash.1 because the annotations in the pull request description changed
(with .werft/ from main)

[werft-gitpod-dev-com]

started the job as gitpod-build-aledbf-stash.2 because the annotations in the pull request description changed
(with .werft/ from main)

[kylos101]

• Open a workspace using https://github.com/aledbf/supervisor-bug (Git Initializer)
• start a regular workspace (flask)
• Restart a previously stopped workspace
• Open a workspace from prebuild https://aledbf-stash.preview.gitpod-dev.com/#prebuild/https://github.com/aledbf/supervisor-bug (Prebuild Initializer) <-- this doesn't appear to be using the Prebuild Initializer 🤔 , the message I get isn't Loading Prebuild. Will check to see how we can do this better in Preview Environments.
• start a private repo (I tested with workspace-preview)

[kylos101]

Tested prebuild initializer fix here: https://www.loom.com/share/c20c82070c3b4e27a75d551cf84b40d8