various: add a fixed name to log about plugin execution

Function execute_directories logged in a way that was meaningless
without additional context:
  systemd[1]: No executables found.
In execute_strv this was partially rectified by extracting the directory
name from one of the directories and using this as the identifier. But
the directory name is not always meaningful, and can also be set from
an environment variable. Let's simplify things by providing a fixed name
that can be used consistently in all log messages. In particular this will
make error messages easier to understand if users report just the error
without additional context.

src/kernel-install/kernel-install.c

@@ -1061,7 +1061,7 @@ static int context_execute(Context *c) {
         }
 
         ret = execute_strv(
-                        /* name = */ NULL,
+                        "plugins",
                         c->plugins,
                         /* root = */ NULL,
                         USEC_INFINITY,