Remove cython workaround for msgpack

Dockerfile

@@ -50,17 +50,11 @@ RUN --mount=type=bind,from=source,target=/mnt/source \
   tar --extract --auto-compress --file=/mnt/source/borgbackup-${BORG_VERSION}.tar.gz --strip-components=1
 
 # Build and Install: Wheel for BorgBackup from source (and cache PIP and GIT repo across builds)
-ARG PIP_CACHE_DIR PIP_DISABLE_PIP_VERSION_CHECK PIP_ROOT_USER_ACTION
+ARG PIP_CACHE_DIR PIP_CONSTRAINT PIP_DISABLE_PIP_VERSION_CHECK PIP_ROOT_USER_ACTION
 ARG PIP_NO_BINARY=:all:
 ARG PIP_USE_FEATURE=no-binary-enable-wheel-cache
 ARG NO_CYTHON_COMPILE=true
 
-# Borg 2.0.0b7 depends on Cython==3.0.2 which cannot be used to compile msgpack<1.0.6
-# Compile before applying PIP_CONSTRAINT
-RUN --mount=type=cache,target=${PIP_CACHE_DIR} --mount=type=tmpfs,target=/tmp \
-  test "${BORG_VERSION}" != "2.0.0b7" || pip install msgpack
-
-ARG PIP_CONSTRAINT
 WORKDIR ${BORG_WHEEL_DIR}
 RUN --mount=type=cache,target=${PIP_CACHE_DIR} --mount=type=tmpfs,target=/tmp \
   pip install pkgconfig && \