Reset tmpDir in gexec.CleanupBuildArtifacts

If tmpDir is not being reset, subsequent calls to gexec.Build will fail.
This is a problem when Build and CleanupBuildArtifacts are being used in
BeforeEach and AfterEach blocks that are being executed multiple times.

gexec/build.go

diff --git a/gexec/build.go b/gexec/build.go
index 3e9bf9f..25b7d51 100644
--- a/gexec/build.go
+++ b/gexec/build.go
@@ -62,6 +62,7 @@
 func CleanupBuildArtifacts() {
 	if tmpDir != "" {
 		os.RemoveAll(tmpDir)
+		tmpDir = ""
 	}
 }
 

gexec/build_test.go

diff --git a/gexec/build_test.go b/gexec/build_test.go
new file mode 100644
index 0000000..7bd62fe
--- /dev/null
+++ b/gexec/build_test.go
@@ -0,0 +1,37 @@
+package gexec_test
+
+import (
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+	"github.com/onsi/gomega/gexec"
+)
+
+var _ = Describe(".Build", func() {
+	var packagePath = "./_fixture/firefly"
+
+	Context("when there have been previous calls to Build", func() {
+		BeforeEach(func() {
+			_, err := gexec.Build(packagePath)
+			Ω(err).ShouldNot(HaveOccurred())
+		})
+
+		It("compiles the specified package", func() {
+			compiledPath, err := gexec.Build(packagePath)
+			Ω(err).ShouldNot(HaveOccurred())
+			Ω(compiledPath).Should(BeAnExistingFile())
+		})
+
+		Context("and CleanupBuildArtifacts has been called", func() {
+			BeforeEach(func() {
+				gexec.CleanupBuildArtifacts()
+			})
+
+			It("compiles the specified package", func() {
+				var err error
+				fireflyPath, err = gexec.Build(packagePath)
+				Ω(err).ShouldNot(HaveOccurred())
+				Ω(fireflyPath).Should(BeAnExistingFile())
+			})
+		})
+	})
+})