This build has a large Chrome uprev in it, so I'm guessing the root cause is probably on the Chrome side: https://chromium.googlesource.com/chromium/src/+log/73.0.3644.0..73.0.3654.0?n=10000

The first failing command in the log looks like this:

01/02 17:11:07.961 INFO |             utils:0287| [stdout] [17:11:07.359] External command failed: exit status 1
01/02 17:11:08.043 INFO |             utils:0287| [stdout] [17:11:07.359] Command: vsh --vm_name=termina --target_container=penguin --owner_id=75435538fefd2b4265232ba7add4ce151dcbd4b2 -- cat /mnt/chromeos/MyFiles/Downloads/shared/hello.txt
01/02 17:11:08.043 INFO |             utils:0287| [stdout] [17:11:07.359] Uncaptured output:
01/02 17:11:08.043 INFO |             utils:0287| [stdout] [17:11:07.360] Error at crostini_files.go:240: Failed to run cat /mnt/chromeos/MyFiles/Downloads/shared/hello.txt: exit status 1
01/02 17:11:08.043 INFO |             utils:0287| [stdout] [17:11:07.360] Stack trace:
01/02 17:11:08.043 INFO |             utils:0287| [stdout] Failed to run cat /mnt/chromeos/MyFiles/Downloads/shared/hello.txt
01/02 17:11:08.044 INFO |             utils:0287| [stdout] 	at chromiumos/tast/local/bundles/cros/vm.verifyFileInContainer (crostini_files.go:240)
01/02 17:11:08.044 INFO |             utils:0287| [stdout] 	at chromiumos/tast/local/bundles/cros/vm.testShareFiles (crostini_files.go:162)
01/02 17:11:08.044 INFO |             utils:0287| [stdout] 	at chromiumos/tast/local/bundles/cros/vm.CrostiniFiles (crostini_files.go:65)
01/02 17:11:08.050 INFO |             utils:0287| [stdout] 	at chromiumos/tast/testing.(*Test).Run.func4 (test.go:228)
01/02 17:11:08.050 INFO |             utils:0287| [stdout] 	at chromiumos/tast/testing.runStages.func1.1 (stage.go:39)
01/02 17:11:08.050 INFO |             utils:0287| [stdout] 	at chromiumos/tast/testing.runAndRecover.func1 (stage.go:69)
01/02 17:11:08.050 INFO |             utils:0287| [stdout] 	at runtime.goexit (asm_amd64.s:2361)
01/02 17:11:08.050 INFO |             utils:0287| [stdout] exit status 1