build: Explicitly Install assets.txt in openedx image

From the upstream Paver DEPR [1], this accounts for a change
in the requirements files:

> Starting in Sumac, these dependencies will be removed from
> requirements/edx/base.txt. Instead, operators will need to install:
> * requirements/edx/assets.txt to build static assets

In the future, we could optimize the openedx image build by installing
assets.txt in a separate, rarely-invalidated cache layer. libsass
in particular takes 60+ seconds to install, so this is promising.
However, for now, in order to prepare for [1], we make just the
simplest possible change, which is to install assets.txt along with
base.txt.

We also take this opportunity to bump the nodeenv version from 1.8.0 to
1.9.1, matching edx-platform's. This is another area where we could make
use of assets.txt in a future Dockerfile refactoring.

[1] openedx/edx-platform#34467

tutor/templates/build/openedx/Dockerfile

@@ -92,10 +92,11 @@ RUN --mount=type=cache,target=/openedx/.cache/pip,sharing=shared \
     # https://pypi.org/project/wheel/
     setuptools==69.1.1 pip==24.0 wheel==0.43.0
 
-# Install base requirements
+# Install base requirements and asset-building requirements
 RUN --mount=type=bind,from=edx-platform,source=/requirements/edx/base.txt,target=/openedx/edx-platform/requirements/edx/base.txt \
+    --mount=type=bind,from=edx-platform,source=/requirements/edx/assets.txt,target=/openedx/edx-platform/requirements/edx/assets.txt \
     --mount=type=cache,target=/openedx/.cache/pip,sharing=shared \
-    pip install -r /openedx/edx-platform/requirements/edx/base.txt
+    pip install -r /openedx/edx-platform/requirements/edx/base.txt -r /openedx/edx-platform/requirements/edx/assets.txt
 
 # Install extra requirements
 RUN --mount=type=cache,target=/openedx/.cache/pip,sharing=shared \
@@ -126,8 +127,8 @@ FROM python AS nodejs-requirements
 ENV PATH=/openedx/nodeenv/bin:/openedx/venv/bin:${PATH}
 
 # Install nodeenv with the version provided by edx-platform
-# https://github.com/openedx/edx-platform/blob/master/requirements/edx/base.txt
-RUN pip install nodeenv==1.8.0
+# https://github.com/openedx/edx-platform/blob/master/requirements/edx/assets.txt
+RUN pip install nodeenv==1.9.1
 RUN nodeenv /openedx/nodeenv --node=18.20.1 --prebuilt
 
 # Install nodejs requirements