[test] Fix OS config for odroids

The config was mistakenly switched to Bionic already. We keep it
unspecified now to prepare reimaging the bots.

No-Try: true
Bug: chromium:1199430
Change-Id: Ib09d9abbe890fc891af69b6630bf21be85328196
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2853586
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74222}

infra/testing/builders.pyl

@@ -608,7 +608,7 @@
     'swarming_dimensions' : {
       'cores': '8',
       'cpu': 'armv7l-32-ODROID-XU4',
-      'os': 'Ubuntu-18.04',
+      'os': 'Ubuntu',
     },
     'swarming_task_attrs': {
       # Use same prio as CI due to limited resources.
@@ -1577,7 +1577,7 @@
     'swarming_dimensions': {
       'cores': '8',
       'cpu': 'armv7l-32-ODROID-XU4',
-      'os': 'Ubuntu-18.04',
+      'os': 'Ubuntu',
     },
     'swarming_task_attrs': {
       'expiration': 21600,
@@ -1606,7 +1606,7 @@
     'swarming_dimensions': {
       'cores': '8',
       'cpu': 'armv7l-32-ODROID-XU4',
-      'os': 'Ubuntu-18.04',
+      'os': 'Ubuntu',
     },
     'swarming_task_attrs': {
       'expiration': 21600,
@@ -1634,7 +1634,7 @@
     'swarming_dimensions': {
       'cores': '8',
       'cpu': 'armv7l-32-ODROID-XU4',
-      'os': 'Ubuntu-18.04',
+      'os': 'Ubuntu',
     },
     'swarming_task_attrs': {
       'expiration': 21600,