fcos.upgrade.basic test consistently failing on aarch64/GCP

[prestist]

fcos.upgrade.basic/upgrade-from-previous is consistently failing on aarch64 GCP instances (europe-west4-a) with SSH reboot timeout errors. The test fails after ~23s on both the initial run and rerun.

Failing builds (all kola-gcp, all aarch64):

• #1136 - testing-devel 44.20260624.20.1
• #1138 - testing-devel 44.20260624.20.1
• #1141 - rawhide 45.20260624.91.5
• #1143 - testing-devel 44.20260626.20.1

Error:

basic.go:186: failed waiting for machine reboot: waiting for reboot failed: : ssh: unexpected packet in response to channel open: <nil>:
basic.go:186: failed waiting for machine reboot: waiting for reboot failed: : EOF:

Key observations:

• The same build (44.20260626.20.1, aarch64) passes on AWS (kola-aws #1101) where upgrade-from-previous takes 223s
• The same build passes on GCP x86_64 (kola-gcp #1142)
• The test was already flaky on GCP aarch64 — the last good build #1125 (2026-06-19) also failed on first run (24.79s) but recovered on rerun (62.69s)
• Since 2026-06-24 reruns also consistently fail (~23s), making the overall build fail
• No relevant code changes between last good and first failing build

This appears to be a GCP platform issue with aarch64 instance reboot timing.