x/build: reverse mac builder GOROOT issue

[bradfitz]

The Mac reverse buildlets are now saying:

Sep 15 22:35:55 farmer.c.symbolic-datum-552.internal coordinator[6215]: 2015/09/15 22:35:55 {darwin-amd64-10_10 a1aafdbe28645bd2161031636739f467e4f16ac9  } failed: runTests: distTestList: Remote
 error: exit status 2, go: cannot find GOROOT directory: /private/var/folders/9l/46n0b92j78n_zn7pq7r1xq800000gn/T/workdir/go
Sep 15 22:35:57 farmer.c.symbolic-datum-552.internal coordinator[6215]: 2015/09/15 22:35:57 {darwin-amd64-10_10 731bdc51157fd7f685fb73c298e97922318ac453  } failed: runTests: distTestList: Remote
 error: exit status 2, go: cannot find GOROOT directory: /private/var/folders/9l/46n0b92j78n_zn7pq7r1xq800000gn/T/workdir/go
Sep 15 22:35:57 farmer.c.symbolic-datum-552.internal coordinator[6215]: 2015/09/15 22:35:57 {darwin-amd64-10_10 211cdf1e004b35e817935537671f9577066800df  } failed: runTests: distTestList: Remote
 error: exit status 2, go: cannot find GOROOT directory: /private/var/folders/q8/bf_4b1ts2zj0l7b0p1dv36lr0000gp/T/workdir/go

Investigate, fix.

/cc @adg @crawshaw

[bradfitz]

Oh, because of snapshots.

The GOROOT is baked into the binaries at one location, and then another machine runs tests and the GOROOT is wrong.

I remember hitting this with the Scaleway ARM builders too but eventually just sidestepped the issue by making the directory to always be /workdir.

I'll see if I can fix it better here.

[gopherbot]

CL https://golang.org/cl/14588 mentions this issue.