env-file: port write_env_file() to label_ops_pre()

Let's make more use of label_ops_pre()/label_ops_post(), and replace
write_env_file_label() by a flag to write_env_file().

This simplifies and normalizes the code.

This also makes one relevant change: it sets the new
WRITE_ENV_FILE_LABEL flag in firstboot.c when we write locale.conf,
where we previously did not (but should have). This should address one
detail of #37857.

src/firstboot/firstboot.c

@@ -448,7 +448,12 @@ static int process_locale(int rfd) {
 
         locales[i] = NULL;
 
-        r = write_env_file(pfd, f, NULL, locales);
+        r = write_env_file(
+                        pfd,
+                        f,
+                        /* headers= */ NULL,
+                        locales,
+                        WRITE_ENV_FILE_LABEL);
         if (r < 0)
                 return log_error_errno(r, "Failed to write /etc/locale.conf: %m");