Add working-directory support

[0x2b3bfa0]

This pull request adds working-directory support, very useful for monorepo setups where paths have to be relative to a subdirectory.

[scruplelesswizard]

Closing and reopening to trigger CI. Sorry for the noise!

[scruplelesswizard]

Useful feature for monorepo setups, but there are two bugs in the current diff that mean it doesn't actually work yet:

1. The guard checks ${{ inputs.working.directory }} (dot) instead of ${{ inputs.working-directory }} (hyphen, matching the input's actual name). inputs.working.directory doesn't resolve to anything, so this condition is always false and the cd never runs — the feature is currently dead code.
2. Even with #1 fixed, split_tests is downloaded via curl into the job's original working directory in the install step. If this step cds into a subdirectory first, ./split_tests on the next line won't resolve — the binary isn't there. You'd need an absolute path to the installed binary (e.g. capture $GITHUB_WORKSPACE/wherever it's installed before the cd, or invoke it via an absolute path) for this to work from a non-default working directory.

Fix the input reference and the binary path and this should be good to merge — happy to take another look once updated.