Archive build script: fix compatibility with older tar on CentOS 7

On CentOS 7, `tar --use-compress-program='xz -9'` tries to run `xz -9` as
executable, rather than running `xz` with `-9` as argument. Passing the
`-9` option via the `XZ_OPT` environment variable, as suggested by
@campbellbarton in D6138, works fine.

build_files/package_spec/build_archive.py

@@ -49,11 +49,14 @@ try:
     if not os.path.exists(output_dir):
         os.mkdir(output_dir)
 
+    archive_env = os.environ.copy()
+
     if extension == 'zip':
         archive_cmd = ['zip', '-9', '-r', package_archive, package_dir]
     elif extension == 'tar.xz':
         archive_cmd = ['tar', '-cf', package_archive, '--owner=0', '--group=0',
-                       '--use-compress-program=xz -9', package_dir]
+                       '--use-compress-program=xz', package_dir]
+        archive_env['XZ_OPT'] = '-9'
     else:
         sys.stderr.write('Unknown archive extension: ' + extension)
         sys.exit(-1)