Invoke chmod once per directory to fix permissions

[christopher-hopper]

Change proposed

I noticed that fix-permissions was traversing the file system twice to set the file mode bits on directories. We can set all file mode bits for both directories and files at the same time.

We could select file mode letter capital X - see chmod man page

(X) : execute (or search for directories) only if the file is a directory or already has execute permission for some user

This doesn't change build speed by much but I thought I'd share it anyway. It simplifies the permissions change using a common, long supported method.

[tobybellwood]

Thanks @christopher-hopper - just trying to untangle some downstream issues - whilst this pr makes 100% sense, it also removes an inadvertent loophole - the final line of the old script will usually exit 0, even if the previous two fail, and thus pass that fix-permissions stage in a dockerfile 😢, whereas this version isn't as forgiving!

Obviously the error here is in whatever dockerfile thinks it is fixing-permissions and isn't, but we just need to work out where those are!