4.x: Scylla IT steps re-resolve the version under the default LATEST

[nikagra]

In .github/workflows/tests@v1.yml (branch scylla-4.x), the two Scylla steps that invoke make
pass only the resolved value, never the matrix value:

- name: Download Scylla (${{ steps.scylla-version.outputs.value }}) image
  env:
    SCYLLA_VERSION_RESOLVED: ${{ steps.scylla-version.outputs.value }}
  run: make download-scylla

- name: Run integration tests on Scylla (...)
  env:
    SCYLLA_VERSION_RESOLVED: ${{ steps.scylla-version.outputs.value }}
  run: make test-integration-scylla

Both download-scylla and test-integration-scylla declare resolve-scylla-version as a
prerequisite, and each step is a fresh make process, so the resolver re-runs in both — under the
Makefile default SCYLLA_VERSION ?= LATEST rather than the lane's actual matrix value. Each step
therefore performs a redundant Docker Hub lookup and writes /tmp/scylla-version-LATEST.resolved
instead of the file keyed to the matrix entry.

The outcome is currently correct, because SCYLLA_VERSION_RESOLVED from the step env wins over the
cache file, so this is waste rather than a wrong version. The Get scylla version step and both
Cassandra steps already pass their matrix value.

Fix: add SCYLLA_VERSION: ${{ matrix.scylla-version }} to both steps.

While here: Makefile:17 defaults GET_VERSION_VERSION ?= 0.4.3 while the workflow overrides it to
0.4.5 in three places — worth aligning so a local run uses the same resolver as CI.

Found while auditing version resolution for DRIVER-924.