fix(action): remove ${{ github.action_path }} literal from input description

GitHub Actions' template parser evaluates expressions inside input
`description` blocks and rejects the `github` context there
(only `inputs`, `env`, etc. are allowed in input metadata).
The literal `${{ github.action_path }}` in the version input's
description tripped: 'Unrecognized named-value: github' and bricked
the entire action-selftest workflow with 'Failed to load action.yml'.

Reworded the description to refer to the install path in plain prose;
the actual `pip install -e "${{ github.action_path }}"` call in
the runs.steps block (where it IS allowed) is unchanged.

Author: claude

action.yml

@@ -32,7 +32,7 @@ inputs:
       Empty (default)  - `pip install pwned-deps` (latest from PyPI).
       "1.2.3"          - `pip install pwned-deps==1.2.3` (pinned).
       "from-source"    - install the action's own checkout via
-                         `pip install -e ${{ github.action_path }}`.
+                         pip install -e <action checkout path>.
                          Used by this repo's action self-test before
                          first PyPI publish, and by forks doing local
                          development against an unreleased revision.