std: treat ENFILE as transient in the pidfd support probe

[valentynkit]

The pidfd support probe special-cases EMFILE from pidfd_open: it returns the
error without caching anything, so the next spawn re-probes. ENFILE falls
through instead, into the fallback arm, so the probe caches pidfd as unsupported
for the rest of the process, even after descriptors free up.

Both errnos come from the same pidfd_open call and mean the same thing: the
process is out of file descriptors, just per-process (EMFILE) vs system-wide
(ENFILE). I don't see a reason to treat them differently here, so this handles
ENFILE the same way:

Err(e) if matches!(e.raw_os_error(), Some(libc::EMFILE | libc::ENFILE)) => {

I kept the raw raw_os_error() check rather than ErrorKind::TooManyOpenFiles
(#158326, which maps both) to match the rest of this probe, but can switch if
you'd prefer.

I didn't add a test, since triggering it needs real fd exhaustion during the
probe, which isn't practical to reproduce. The EMFILE arm isn't tested either.

r? libs