[REVIEW] Fix jitcache-multiprocess test failure, fix CUDA 11 related CI issues

[harrism]

Recent changes to rmm::mr::get_default_resource() caused the CUDA context
to be created in the JITCACHE_MULTIPROCESS_TEST before the child process is
forked, which caused the child to not get a context. This PR fixes the failure.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov]

Codecov Report

Merging #5942 into branch-0.15 will decrease coverage by 4.21%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.15    #5942      +/-   ##
===============================================
- Coverage        88.65%   84.43%   -4.22%     
===============================================
  Files               57       80      +23     
  Lines            10945    13424    +2479     
===============================================
+ Hits              9703    11335    +1632     
- Misses            1242     2089     +847     

Impacted Files	Coverage Δ
python/cudf/cudf/core/dtypes.py	86.84% <0.00%> (-10.09%)	⬇️
python/cudf/cudf/core/abc.py	91.48% <0.00%> (-2.96%)	⬇️
python/cudf/cudf/core/frame.py	89.76% <0.00%> (-1.42%)	⬇️
python/cudf/cudf/utils/ioutils.py	86.13% <0.00%> (-1.21%)	⬇️
python/dask_cudf/dask_cudf/sorting.py	93.12% <0.00%> (-0.99%)	⬇️
python/cudf/cudf/core/indexing.py	95.22% <0.00%> (-0.68%)	⬇️
python/cudf/cudf/utils/dtypes.py	85.89% <0.00%> (-0.54%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	93.18% <0.00%> (-0.10%)	⬇️
python/cudf/cudf/io/avro.py	81.81% <0.00%> (ø)
python/cudf/cudf/core/ops.py	77.58% <0.00%> (ø)
... and 61 more

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[galipremsagar]

rerun tests

[devavret]

Jitcache changes LGTM