Bsdtar has no explicit flag for xz decompression

At least as of Mac OS 10.13.6 the included bsdtar has no flag for xz
decompression while `z` means gzip decompression which is obviously
problematic for xz compressed archives.

I believe that any tar version on Mac OS that can decompress xz archives
also automatically detects compression formats. So `tar xf` should be
fine.

Fixes #119

ghcup

@@ -1038,8 +1038,8 @@ unpack() {
     case "${file_ext}" in
         xz)
             if test "${mydistro}" = "darwin"; then
-              debug_message "tar -xzf \"${filename}\""
-              ( tar -xzf "${filename}" ) || die "unpacking failed!"
+              debug_message "tar xf \"${filename}\""
+              ( tar xf "${filename}" ) || die "unpacking failed!"
             else
               debug_message "xz -cd \"${filename}\" | tar -xf -"
               ( xz -cd "${filename}" | tar -xf - ; ) || die "unpacking failed!"