Working directory should be CI_PROJECT_DIR

Problem

The initial working directory is /builds. This isn't particularly useful, most files referenced by a job are in the project directory.

Proposal

Set the working directory to be $CI_PROJECT_DIR.

## Example

This is the current behaviour with the script keyword:

$ echo CI_PROJECT_DIR is $CI_PROJECT_DIR
CI_PROJECT_DIR is /builds/josephburnett/hello-runner
$ echo CI_BUILDS_DIR is $CI_BUILDS_DIR
CI_BUILDS_DIR is /builds
$ echo pwd is $(pwd)
pwd is /builds/josephburnett/hello-runner

And it causes local step references in .gitlab-ci.yml to fail.