x/build: misc-compile TryBots are too slow again

[bcmills]

https://farmer.golang.org/try?commit=178e9821 (for CL 354149) was stuck running misc-compile-mac-win for over 25 minutes — it was the slowest builder of the lot. My understanding from #32632 is that we aim for TryBots to complete in about 5 minutes — or only ⅕ of that time.

It isn't obvious to me why that particular builder would have been so slow on this run.

CC @golang/release

[dmitshur]

A comment on Apr 23 mentions a revised target time:

Our updated target time for TryBot completion time is 10 minutes (see subject of issue #17104),

But 25 mins still greatly exceeds 10 mins.

We should use our recently restored metrics (#48581) to tell whether this was a one-off occurrence or a more regular problem.

[bcmills]

As another anecdatapoint, the misc-compile and js-wasm TryBots on CL 352810 have so far taken much longer than the -longtest SlowBots.

(js-wasm is tracked separately as #35170.)