Make scripts zsh proof

E.g. [`workon.sh`](https://github.com/faasm/faasm/blob/bfb77185809a03a75113d99fe36b8c58112b7fa8/bin/workon.sh#L12..L15) is zsh proof, but not [`create_venv.sh`](https://github.com/faasm/faasm/blob/main/bin/create_venv.sh#L5). `bash` and `zsh` evaluate the expression for `THIS_DIR` differently.