Allow gexec.Build to be called concurrently

Signed-off-by: Jason Keene <jkeene@pivotal.io>

gexec/build.go

diff --git a/gexec/build.go b/gexec/build.go
index 25b7d51..220c8c4 100644
--- a/gexec/build.go
+++ b/gexec/build.go
@@ -9,9 +9,13 @@
 	"path"
 	"path/filepath"
 	"runtime"
+	"sync"
 )
 
-var tmpDir string
+var (
+	mu     sync.Mutex
+	tmpDir string
+)
 
 /*
 Build uses go build to compile the package at packagePath.  The resulting binary is saved off in a temporary directory.
@@ -60,6 +64,8 @@
 gexec. In Ginkgo this is typically done in an AfterSuite callback.
 */
 func CleanupBuildArtifacts() {
+	mu.Lock()
+	defer mu.Unlock()
 	if tmpDir != "" {
 		os.RemoveAll(tmpDir)
 		tmpDir = ""
@@ -68,6 +74,8 @@
 
 func temporaryDirectory() (string, error) {
 	var err error
+	mu.Lock()
+	defer mu.Unlock()
 	if tmpDir == "" {
 		tmpDir, err = ioutil.TempDir("", "gexec_artifacts")
 		if err != nil {