Add `FLOKI_WORKING_DIR` environment variable

[maxdymond]

• Add FLOKI_WORKING_DIR environment variable that is accessible inside the container. Can be used when mount: has been specified in floki.yaml.

Why this change?

If someone specifies mount: then the assumption that code is mounted at /src is wrong.

Relevant testing

Local testing

Contributor notes

N/A

Checks

These aren't hard requirements, just guidelines

• [x] New/modified Rust code formatted with cargo fmt
• [x] Documentation and README.md updated for this change, if necessary
• [x] CHANGELOG.md updated for this change

[maxdymond]

Reviewed internally.