nut: fix install (#18848)

* nut: fix install

* flake8

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

@@ -38,7 +38,7 @@ def install(self, spec, prefix):
         install('README.md', prefix)
         mkdirp(prefix.bin)
         mkdirp(prefix.lib)
-        install('../spack-build/apps/bh-3', prefix.bin)
-        install('../spack-build/lib/libnut.a', prefix.lib)
+        install(join_path(self.build_directory, 'apps/bh-3'), prefix.bin)
+        install(join_path(self.build_directory, 'lib/libnut.a'), prefix.lib)
         install_tree('test/data', prefix.data)
         install_tree('lib', prefix.include)