cmd/dist: set buildvcs=false if not within a git repository

When compiling without a Git repository, "go install" returns an error:

error obtaining VCS status: exit status 128
	Use -buildvcs=false to disable VCS stamping.

Set -buildvcs=false in go install when isGitDir() returns false.

Signed-off-by: Christian Stewart <christian@aperture.us>

src/cmd/dist/buildtool.go

@@ -213,6 +213,9 @@ func bootstrapBuildTools() {
 	if tool := os.Getenv("GOBOOTSTRAP_TOOLEXEC"); tool != "" {
 		cmd = append(cmd, "-toolexec="+tool)
 	}
+	if !isGitRepo() {
+		cmd = append(cmd, "-buildvcs=false")
+	}
 	cmd = append(cmd, "bootstrap/cmd/...")
 	run(base, ShowOutput|CheckExit, cmd...)