Handle readonly directories in tmpdir

Some tests make their temporary directories readonly. If they fail or
crash before cleaning up, they could leave these readonly directories
behind with files in them. os.RemoveAll fails with an error in this
case, and we can't start the build until they're removed.

Test: `m blueprint_tools` to run the new go tests
Change-Id: I761f96579e96167ebfd98c6cca59765bd50536ec