Add nightly version for ucx-py in ci script (#8419)

`ucx-py` recently reverted back to old versioning format from calVer. This PR hard-codes the nightly version.

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Dillon Cullinan (https://github.com/dillon-cullinan)
  - https://github.com/jakirkham
  - AJ Schmidt (https://github.com/ajschmidt8)

URL: #8419

ci/gpu/build.sh

@@ -80,7 +80,7 @@ gpuci_conda_retry install -y \
                   "rapids-notebook-env=$MINOR_VERSION.*" \
                   "dask-cuda=${MINOR_VERSION}" \
                   "rmm=$MINOR_VERSION.*" \
-                  "ucx-py=${MINOR_VERSION}"
+                  "ucx-py=0.21.*"
 
 # https://docs.rapids.ai/maintainers/depmgmt/
 # gpuci_conda_retry remove --force rapids-build-env rapids-notebook-env