x/build: restore performance benchmarking dashboard

[ianlancetaylor]

In a trybot run in https://storage.googleapis.com/go-build-log/dd2f2f0c/linux-amd64_19b3a9d1.log , after the build and the tests are complete, I see this:

##### 
iteration: 0
start-time: 2017-06-02T00:53:45Z
building testgo failed: exit status 1
can't load package: package .: no buildable Go source files in /tmp/workdir/go-parent

Error: tests failed: dist test failed: bench:bench-cmd-go.exe -test.bench . -test.benchmem -test.run ^$ -test.benchtime 100ms: exit status 2

[ianlancetaylor]

Another trybot run: https://storage.googleapis.com/go-build-log/f7eb06b6/linux-amd64_26eecad0.log

[bradfitz]

@quentinmit, this looks like the new benchmarks-on-trybot code having problems in some cases. Can you take a look?

[bradfitz]

Another:
https://storage.googleapis.com/go-build-log/12c54536/linux-amd64_c57e3046.log
From:
https://go-review.googlesource.com/c/46030/

@quentinmit ?

[bradfitz]

"use of internal package not allowed"
https://storage.googleapis.com/go-build-log/8a70806e/linux-amd64_ca9bc8f3.log

[bradfitz]

Trybot benchmarking was disabled, as announced on golang-dev, so pushing this to Go 1.10.