x/build: make linux-amd64-noopt builder also a pre-submit TryBot

[josharian]

CL 37751 would have benefitted from this.

[kevinburke]

Does this help at all? https://go-review.googlesource.com/c/37146/

[mundaym]

This would also have caught CL 36839.

[josharian]

Also CL 38087.

[kevinburke]

Here are some more tests for coordinator that may also be helpful - https://go-review.googlesource.com/c/39292/

[josharian]

Also CL 36879

[bcmills]

Also CL 263097.

[dmitshur]

I'm not working on this at this time, so unassigning.

CC @toothrot, @cagedmantis.