Upgrade build scripts to go1.21

We needed a way to generate draft releases for git-sizer binaries.

This commit adds a new `.github/workflows/release.yml` github action
that will generate a draft release when a new tag version is pushed.

the action will be triggered After the tag is created and pushed using:

```
git tag -as v$VERSION
git push origin v$VERSION
```

…-upload

Generate automatic draft release

The name `gitDir` is less ambiguous. Also rename method `Path()` to
`GitDir()`.

Add a method `Repository.GitPath(relPath)`, which invokes `git
rev-parse --git-path $relPath` to find the path to a file within the
Git repository.

In `NewRepository()`, instantiate the `Repository` object earlier so
that the new method can be used to find the path to `shallow`.

Extract a method to determine whether the repository seems to be a
full clone. Call it from `NewRepository()`.

If you already have the desired `GIT_DIR`, there's no need to
determine it from the current path.

There's no need to deduce the `GIT_DIR` for a bare repository.

Update the installation steps to use 'go install'

As of Git v2.38.0, there is an option to prevent Git from accessing bare
repositories unless asked for explicitly (via `--git-dir` or `GIT_DIR`):
`safe.bareRepository`.

The tests of `git sizer`, however, assume that Git will access a bare
repository when the current directory points inside that repository.
This only works if `safe.bareRepository` indicates that this is safe.

If that is not the case, i.e. if `safe.bareRepository` is set to
`explicit`, Git demands that the environment variable `GIT_DIR` is set
(either explicitly, or via `--git-dir`) when accessing bare
repositories.

So let's set `GIT_DIR` for the test cases that work on bare
repositories.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Improve repository detection and deal with `safe.bareRepository=explicit`