Example CL: https://chromium-review.googlesource.com/c/chromium/tools/build/+/1419877

Failure error in the trigger build:

Traceback (most recent call last):
  File "/b/swarming/w/ir/cache/builder/src/tools/mb/mb.py", line 63, in Main
    ret = self.args.func()
  File "/b/swarming/w/ir/cache/builder/src/tools/mb/mb.py", line 276, in CmdAnalyze
    vals = self.Lookup()
  File "/b/swarming/w/ir/cache/builder/src/tools/mb/mb.py", line 615, in Lookup
    config = self.ConfigFromArgs()
  File "/b/swarming/w/ir/cache/builder/src/tools/mb/mb.py", line 697, in ConfigFromArgs
    (self.args.builder, self.args.master, self.args.config_file))
MBErr: Builder name "linux-rel"  not found under masters[tryserver.chromium.linux] in "/b/swarming/w/ir/cache/builder/src/tools/mb/mb_config.pyl"
step returned non-zero exit code: 1