fontc cannot compile font sources in /tmp

On Ubuntu 20.04, with e.g. an example font generated by https://gist.github.com/madig/f07d821171338062b3d132f4ccfbeb6c#file-generate-py and saved to /tmp/test.ufo, running fontc fails:

```
❯ target/release/fontc -b /tmp/fontc-build/ /tmp/test.ufo/
Font IR error: 'IO failure: 'Permission denied (os error 13)''
```

Running it with sudo works. It also works when you move the UFO to a subfolder:

```
❯ mkdir -p /tmp/aaa/

❯ mv /tmp/test.ufo/ /tmp/aaa/

❯ target/release/fontc -b /tmp/fontc-build/ /tmp/aaa/test.ufo/

```

Interestingly, setting /tmp as the build folder will still fail:

```
❯ target/release/fontc -b /tmp/ /tmp/aaa/test.ufo/
IO error: 'Permission denied (os error 13)'
```

Reproduce the failure on Ubuntu 20.04 using the reported /tmp/test.ufo input and target/release/fontc commands, then inspect fontc's handling of source and build paths under /tmp. Done means compiling the UFO from /tmp and using /tmp as the build folder without a permission error, while preserving the working nested-directory case.

Written by the indexing model from the issue text.