Could not build PyCall (name clash for `/tmp/installer.sh`)

While building PyCall on a shared workstation I encountered this error:

```
ERROR: Error building `PyCall`:
┌ Info: Using the Python distribution in the Conda package by default.
└ To use a different Python version, set ENV["PYTHON"]="pythoncommand" and re-run Pkg.build("PyCall").
[ Info: Downloading miniconda installer ...
ERROR: LoadError: IOError: unlink("/tmp/installer.sh"): operation not permitted (EPERM)
```

The problem was that the file `/tmp/installer.sh` already existed, and it belonged to a different user.