proposal: x/build: add a longtest builder for Windows

[bcmills]

The builder dashboard currently includes a longtest builder for linux/amd64.

It seems like it would be wasteful to add longtest builders for every supported OS, but Windows in particular is different enough from Linux that it might catch real bugs — and many of us working on the core project do not test explicitly on Windows nearly as often as we ought to (see, for example, #25300).

I propose that we add a Windows builder that runs tests in long mode.

See also #29641, #29252, #29266.

CC @dmitshur @bradfitz @alexbrainman

[bcmills]

Dup of #26529. (GitHub search has failed me again!)