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/basic/env-file.h

@@ -15,7 +15,11 @@ int load_env_file_pairs_fd(int fd, const char *fname, char ***ret);
 
 int merge_env_file(char ***env, FILE *f, const char *fname);
 
-int write_env_file(int dir_fd, const char *fname, char **headers, char **l);
+typedef enum WriteEnvFileFlags {
+        WRITE_ENV_FILE_LABEL = 1 << 0,
+} WriteEnvFileFlags;
+
+int write_env_file(int dir_fd, const char *fname, char **headers, char **l, WriteEnvFileFlags flags);
 
 int write_vconsole_conf(int dir_fd, const char *fname, char **l);