[infra] Run gcc-bot tests on Focal image

We don't build with bundled sysroot on gcc builders, hence we need to
match the sysroot on tester bots. There is already a number of
Focal bots in chromium.tests and the capacity needed for these gcc
bots is insignificant.

No-Try: true
Bug: chromium:1307180
Change-Id: Id9d3d08bdb09923cf919b17286c949c6ac9005ed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3747872Reviewed-by: Alexander Schulze <alexschulze@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81563}

infra/testing/builders.pyl

@@ -422,7 +422,7 @@
   },
   'v8_linux64_gcc_rel_ng_triggered': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-18.04',
+      'os': 'Ubuntu-20.04',
     },
     'tests': [
       {'name': 'v8testing'},
@@ -1327,7 +1327,7 @@
   },
   'V8 Linux64 gcc': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-18.04',
+      'os': 'Ubuntu-20.04',
     },
     'tests': [
       {'name': 'v8testing'},