[CI] Pin built OpenAssetIO to mirror PyPI version

Unit tests and Jupyter notebooks use the latest release of OpenAssetIO
(see `examples/resources/requirements.txt` and `pyproject.toml`).

However, C++ tests, which currently must build OpenAssetIO from source,
were using the latest `main`.

So ensure we test against a consistent version of OpenAssetIO for both
C++ and Python tests by pinning the checkout of OpenAssetIO to the
latest release. Note that we cannot specify a semver range, so this
will get out of date on the next release.

Future CI work is required to design a more well-thought testing policy
e.g. testing against supported version ranges vs. `main` and nightlies.

Signed-off-by: David Feltell <david.feltell@foundry.com>

.github/build_openassetio/action.yml

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: Apache-2.0
-# Copyright 2023 The Foundry Visionmongers Ltd
+# Copyright 2023-2024 The Foundry Visionmongers Ltd
 
 # Composite action for reuse within other workflows.
 # Builds OpenAssetIO.
@@ -15,6 +15,7 @@ runs:
       with:
         repository: OpenAssetIO/OpenAssetIO
         path: openassetio-checkout
+        ref: v1.0.0-rc.1.0
 
     - name: Build OpenAssetIO
       shell: bash