Alibuild

This release uses precompiled tarballs by default on CentOS 8, without the user having to specify `--remote-store`.

This release contains significant changes and improvements:

- support for reusing more precompiled packages -- now, packages built with a different `tag:` key in alidist are considered for reuse if the tag they were built from points to the same commit as is being built now
- a fix for uploads when using an s3:// remote store
- a fix in checking for pigz and tar support when building inside a Docker container -- now support is checked inside the container

This pre-release contains a fix for uploads when using an `s3://` remote store.

This pre-release contains experimental support for reusing more precompiled packages -- now, packages built with a different `tag:` key in alidist are considered for reuse if the tag they were built from points to the same commit as is being built now.

This release adds a bit of debug output when uploading tarballs to S3.