cmd/coordinator: restore GOPATH for benchmark builds

CL 442258 removed GOPATH from the environment, as I believed it was
inconsequential. However, it is still used to store downloaded modules.
Without setting GOPATH, the modules end up stored on the root disk
(instead of /workdir tmpfs), which has limited space, resulting in out
of disk space errors.

Change-Id: I0a51e94df07fbe93bbd6e7e4bd7f205a8f77146c
Reviewed-on: https://go-review.googlesource.com/c/build/+/443858
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cmd/coordinator/buildstatus.go

@@ -1262,6 +1262,7 @@ func (st *buildStatus) runBenchmarkTests() (remoteErr, err error) {
 	}
 	goroot := st.conf.FilePathJoin(workDir, "go")
 	baselineGoroot := st.conf.FilePathJoin(workDir, baselineDir)
+	gopath := st.conf.FilePathJoin(workDir, "gopath")
 
 	// Install baseline toolchain in addition to the experiment toolchain.
 	toolchainBaselineCommit, remoteErr, err := st.installBaselineToolchain(goroot, baselineDir)
@@ -1293,6 +1294,7 @@ func (st *buildStatus) runBenchmarkTests() (remoteErr, err error) {
 		"BENCH_BRANCH="+st.RevBranch,
 		"BENCH_REPOSITORY="+repo,
 		"GOROOT="+goroot,
+		"GOPATH="+gopath, // For module cache storage
 		"GOPROXY="+moduleProxy(), // GKE value but will be ignored/overwritten by reverse buildlets
 	)
 	env = append(env, st.conf.ModulesEnv("benchmarks")...)