feat: Add a way to select a specific branch before building the devcontainer

### Description

Improve feature parity with `github codespaces`.

The goal is to add a new `coder_parameter` called `target branch` to workspace templates.
It will let the user select a branch from which to build a devcontainer.

Unlike the devcontainer cli, the `git clone` happen in the `enbuilder` container.
So I think it is the responsibility of `envbuilder` to checkout to the target branch **before building the container**.

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PS: The [workaround in readme.md](https://github.com/coder/envbuilder?tab=readme-ov-file#git-branch-selection) does not work on all platforms.

Read the readme.md Git branch selection workaround first, then trace the coder_parameter handling and the envbuilder checkout flow. Done means a target branch can be selected in workspace templates and is checked out before the devcontainer is built, including platforms where the README workaround fails.

Written by the indexing model from the issue text.