nbdkit: Fix configure arg (#20488)

var/spack/repos/builtin/packages/nbdkit/package.py

@@ -24,3 +24,7 @@ class Nbdkit(AutotoolsPackage):
     depends_on('automake', type='build')
     depends_on('libtool',  type='build')
     depends_on('m4',       type='build')
+
+    def configure_args(self):
+        args = ['bashcompdir={0}'.format(prefix)]
+        return args