[mgs-updates] Adjust RoT bootloader post-update timeout (#8825)

During the RoT bootloader's post_update stage, we are currently giving
[120
seconds](https://github.com/oxidecomputer/omicron/blob/main/nexus/mgs-updates/src/rot_bootloader_updater.rs#L31)
to [poll the RoT to retrieve
boot_info](https://github.com/oxidecomputer/omicron/blob/main/nexus/mgs-updates/src/rot_bootloader_updater.rs#L191-L203),
and then [resetting the device
again](https://github.com/oxidecomputer/omicron/blob/main/nexus/mgs-updates/src/rot_bootloader_updater.rs#L252).
`post_update_timeout()` should align more closely to that.

Author: karencfv

nexus/mgs-updates/src/driver_update.rs

@@ -672,7 +672,7 @@ fn post_update_timeout(update: &PendingMgsUpdate) -> Duration {
         PendingMgsUpdateDetails::RotBootloader { .. } => {
             // Resetting the bootloader requires multiple RoT resets; give this
             // a longer timeout.
-            Duration::from_secs(120)
+            Duration::from_secs(180)
         }
         PendingMgsUpdateDetails::HostPhase1(..) => {
             // Resetting a sled takes several minutes (mostly DRAM training);