Fix container builds by generating podman-compatible source archives
With this change the SOURCE_ARCHIVE goal archives the commit tree object HEAD^{tree} rather than the commit object HEAD. The reason for this is to circumvent a problem with podman/buildah which will currently error out if it finds a global extended header when adding a tar archive.
The git archive command will generate tar archives with a global extended header containing the commit hash as a comment if the archived object is a commit.
Part of agile#1974.