ENH Remove 'rapidsai-nightly' conda channel when building main branch

[jjacobelli]

Remove rapidsai-nightly conda channel when building main branch

[codecov-commenter]

Codecov Report

Merging #1577 (bad553a) into branch-0.20 (250f2c8) will decrease coverage by 1.73%.
The diff coverage is 65.27%.

❗ Current head bad553a differs from pull request most recent head b346350. Consider uploading reports for the commit b346350 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.20    #1577      +/-   ##
===============================================
- Coverage        61.04%   59.31%   -1.74%     
===============================================
  Files               73       77       +4     
  Lines             3219     3367     +148     
===============================================
+ Hits              1965     1997      +32     
- Misses            1254     1370     +116     

Impacted Files	Coverage Δ
python/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/centrality/katz_centrality.py	81.25% <ø> (ø)
python/cugraph/components/connectivity.py	95.65% <ø> (ø)
python/cugraph/dask/centrality/katz_centrality.py	29.16% <ø> (ø)
python/cugraph/dask/link_analysis/pagerank.py	21.87% <0.00%> (ø)
python/cugraph/dask/traversal/bfs.py	25.00% <0.00%> (-2.59%)	⬇️
python/cugraph/structure/number_map.py	61.69% <16.66%> (-2.13%)	⬇️
...ure/graph_implementation/simpleDistributedGraph.py	22.72% <22.72%> (ø)
python/cugraph/structure/symmetrize.py	68.62% <60.00%> (-4.71%)	⬇️
...raph/structure/graph_implementation/simpleGraph.py	73.98% <73.98%> (ø)
... and 34 more

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[BradReesWork]

@gpucibot merge