libstd: Use CLOCK_BOOTTIME for Instant

Discussion is ongoing upstream at
rust-lang/rust#87906
about how exactly to make a change like this, but most of the concerns
(backwards compatibility, legacy kernels prior to 2.6.39, non-Linux
OSes) are not applicable to us.

If upstream decides to keep `Instant` as `CLOCK_MONOTONIC`, we should
figure out an alternate solution for our use of `quiche` before backing
this out.

Bug: 200694560
Change-Id: I3ad265f01064f14cddac815117406c5941a3b2ef
Merged-In: I8f7cd025c1ef9e1c7e89163d7d73220181b748ca

linux-x86/1.51.0/src/stdlibs/library/std/src/sys/unix/time.rs

@@ -293,7 +293,7 @@ mod inner {
 
     impl Instant {
         pub fn now() -> Instant {
-            Instant { t: now(libc::CLOCK_MONOTONIC) }
+            Instant { t: now(libc::CLOCK_BOOTTIME) }
         }
 
         pub const fn zero() -> Instant {