Fix cache key in nx-integration-tests using undefined inputs.ref (#379)

The integration-tests job restored the project cache with:

  key: ${{ runner.os }}-project-${{ inputs.ref }}

But `inputs.ref` is not a defined input for this workflow — the
declared inputs are `repository`, `pr_head`, and `pr_base`. The
expression resolved to an empty string, producing the key
`Linux-project-` on every run, which never matched the entry saved
by the nx-project-setup job (which used `inputs.pr_head` as the
ref throughout).

The result was a guaranteed cache miss on any fresh PR run: the
`./main` directory was empty when the integration-tests job started,
the PHPUnit bootstrap fell back to running `setup:install` against
a `tmp-mysql` container that had never been initialised, and the
entire suite crashed at bootstrap before a single test executed.

Fix: replace `inputs.ref` with `inputs.pr_head`, consistent with
every other place in this workflow that references the head SHA.

Author: thebraziliandeveloper

.github/workflows/nx-integration-tests.yml

@@ -111,7 +111,7 @@ jobs:
         uses: actions/cache/restore@v4
         with:
           path: main
-          key: ${{ runner.os }}-project-${{ inputs.ref }}
+          key: ${{ runner.os }}-project-${{ inputs.pr_head }}
 
       # could probably swap this to a docker container in future
       - name: Install NX