Rely on WORKDIR env var to avoid ugly ternary operators in workflows. (#9456)

Ternary operators in GitHub Actions syntax are pretty ugly and hard to
read: `inputs.working-directory == '' && '.' ||
inputs.working-directory` means "if the condition is true, use `'.'` and
otherwise use the expression after the `||`".

This PR performs the ternary as few times as possible, assigning its
outcome to an env var we can then reuse as needed.

.github/actions/poetry_setup/action.yml

@@ -62,30 +62,18 @@ runs:
       run: |
         poetry lock --check
 
-    - name: Set proper Poetry.lock file
-      shell: bash
-      env:
-        WORKDIR: ${{ inputs.working-directory == '' && '.' || inputs.working-directory }}
-      run: |
-        if [ -f "$WORKDIR/poetry.lock" ]; then
-          echo 'Using working directory poetry.lock in cache key'
-          cp "$WORKDIR/poetry.lock" poetry-lock.cache-key
-        else
-          echo 'Using the top-level poetry.lock in cache key'
-          cp poetry.lock poetry-lock.cache-key
-        fi
-
     - uses: actions/cache@v3
       id: cache-poetry
       env:
         SEGMENT_DOWNLOAD_TIMEOUT_MIN: "15"
+        WORKDIR: ${{ inputs.working-directory == '' && '.' || inputs.working-directory }}
       with:
         path: |
           ~/.cache/pypoetry/virtualenvs
           ~/.cache/pypoetry/cache
           ~/.cache/pypoetry/artifacts
-          ${{ inputs.working-directory == '' && '.' || inputs.working-directory }}/.venv
-        key: poetry-${{ runner.os }}-${{ runner.arch }}-py-${{ inputs.python-version }}-poetry-${{ inputs.poetry-version }}-${{ inputs.cache-key }}-${{ hashFiles('poetry-lock.cache-key') }}
+          ${{ env.WORKDIR }}/.venv
+        key: poetry-${{ runner.os }}-${{ runner.arch }}-py-${{ inputs.python-version }}-poetry-${{ inputs.poetry-version }}-${{ inputs.cache-key }}-${{ hashFiles(format('{0}/poetry.lock', env.WORKDIR)) }}
 
     - run: ${{ inputs.install-command }}
       working-directory: ${{ inputs.working-directory }}