Auto skip issue

Good day,

I'm trying to add requirements.txt file as skip key:
```bash
$ cat Bakefile
localdev/foo: @skip=foo/requirements.txt
pip install -r foo/requirements.txt
$ foo/requirements.txt
```

However for some reason `bake` keeps running `pip install...` every time:


Am I missing something here?

Env: Ubuntu 16.04, Python 3.7.4
```
$ pipenv graph
bake-cli==0.12.0
- click [required: Any, installed: 7.0]
- colorama [required: Any, installed: 0.4.1]
- delegator.py [required: Any, installed: 0.1.1]
- pexpect [required: >=4.1.0, installed: 4.7.0]
- ptyprocess [required: >=0.5, installed: 0.6.0]
- networkx [required: Any, installed: 2.3]
- decorator [required: >=4.3.0, installed: 4.4.0]
- pygments [required: Any, installed: 2.4.2]
```

Reproduce the behavior using the shown Bakefile and foo/requirements.txt on the documented environment, then trace how bake interprets the @skip key. Done means the pip install command is skipped when foo/requirements.txt already exists, while the target still runs when that file is absent.

Written by the indexing model from the issue text.