tree-wide: Remove O_CLOEXEC from fdopen

fdopen doesn't accept "e", it's ignored. Let's not mislead people into
believing that it actually sets O_CLOEXEC.

From `man 3 fdopen`:

> e (since glibc 2.7):
> Open the file with the O_CLOEXEC flag. See open(2) for more information. This flag is ignored for fdopen()

As mentioned by @jlebon in #11131.

src/shared/exec-util.c

@@ -308,7 +308,7 @@ static int gather_environment_consume(int fd, void *arg) {
 
         assert(env);
 
-        f = fdopen(fd, "re");
+        f = fdopen(fd, "r");
         if (!f) {
                 safe_close(fd);
                 return -errno;