git placeholders are not expanded in release tarballs

git placeholders are not expanded in the [libqb-2.0.8.tar.xz](https://github.com/ClusterLabs/libqb/releases/download/v2.0.8/libqb-2.0.8.tar.xz) and [libqb-2.0.8.tar.gz](https://github.com/ClusterLabs/libqb/releases/download/v2.0.8/libqb-2.0.8.tar.gz) release tarballs, suggesting that they were not created by `git archive`.

For example, if I use the automatically-generated [v2.0.8.tar.gz](https://github.com/ClusterLabs/libqb/archive/refs/tags/v2.0.8.tar.gz) tarball that I know from #312 I'm not supposed to use, this line:

https://github.com/ClusterLabs/libqb/blob/40542f314ebe7312fa995677ed5c967150dd5d6a/configure.ac#L9

is expanded to:

```m4
m4_esyscmd([build-aux/git-version-gen $(echo '002171bbc??tag: v2.0.8'\
```

and this line:

https://github.com/ClusterLabs/libqb/blob/40542f314ebe7312fa995677ed5c967150dd5d6a/configure.ac#L816

is expanded to:

```m4
SOURCE_EPOCH='1689939039' # template for rewriting by git-archive
```

However in the release tarballs these `$Format:…$` placeholders remain unchanged.