libxml2: add command property (#31296)

var/spack/repos/builtin/packages/libxml2/package.py

@@ -54,6 +54,10 @@ def url_for_version(self, version):
     patch('nvhpc-configure.patch', when='%nvhpc')
     patch('nvhpc-elfgcchack.patch', when='%nvhpc')
 
+    @property
+    def command(self):
+        return Executable(self.prefix.bin.join('xml2-config'))
+
     @property
     def headers(self):
         include_dir = self.spec.prefix.include.libxml2