On macOS, with the default 'bsdtar', 'make build/<pkg>' fails to create
a tarball, but does not warn about it, outputting:

> ======== Building tarball archive-devel/<pkg-vers>.tar...
> ######## Built new package archive-devel/<pkg-vers>.tar!
> ======== Building tarball archive/<pkg-ver>.tar...
> ######## Built new package archive/<pkg-ver>.tar!

['bsdtar' does not expose '--transform' option. Though it provides a
similar '-s' option with a bit different syntax.]

This patch suggests asserting a tarball was generated. This would
result in a more helpful output on a failed generation:

> ======== Building tarball archive-devel/<pkg-vers>.tar...
> Build error for archive-devel/<pkg-vers>.tar: (cl-assertion-failed (file-readable-p tarball))
> ######## Build of package archive-devel/<pkg-vers>.tar FAILED!!
> ======== Building tarball archive/<pkg-ver>.tar...
> Build error for archive/<pkg-ver>.tar: (cl-assertion-failed (file-readable-p tarball))
> ######## Build of package archive/<pkg-ver>.tar FAILED!!



In GNU Emacs 28.0.90 (build 1, x86_64-apple-darwin21.1.0, NS appkit-2113.00 Version 12.0.1 (Build 21A559))