nsjail: Always remount /tmp before src/out/dist

A user was trying to use OUT_DIR=/tmp/...-out DIST_DIR=/tmp/...-dist

Test: OUT_DIR=/tmp/test-out DIST_DIR=/tmp/test-dist m nothing
Change-Id: I811cc8e1bda8d766406603df47309a8637cef48c

ui/build/sandbox_linux.go

@@ -181,15 +181,15 @@ func (c *Cmd) wrapSandbox() {
 		// For now, just map everything. Make most things readonly.
 		"-R", "/",
 
+		// Mount a writable tmp dir
+		"-B", "/tmp",
+
 		// Mount source are read-write
 		"-B", sandboxConfig.srcDir,
 
 		//Mount out dir as read-write
 		"-B", sandboxConfig.outDir,
 
-		// Mount a writable tmp dir
-		"-B", "/tmp",
-
 		// Disable newcgroup for now, since it may require newer kernels
 		// TODO: try out cgroups
 		"--disable_clone_newcgroup",