Improvements to build.sh, build documentation and conda developer environments [skip-ci]

[dantegd]

Closes #4773

PR adds a few improvements I had made locally to build.sh, conda environments as well as updating our build documentation:

• Updates for build.sh:

  • [x] Not recompile everything on rebuilds
  • [x] Only build correct targets when installing (priorly, even if only libcuml++ wanted to be built, it would also build prims, libcuml_c, etc.)
  • [x] Print total compilation time (in preparation of porting cuDF's ninja log post processing)

• Updates to conda environments:

  • [x] Eliminated all CUDA versions to make maintenance easier, and added instructions to quickly change CUDA version of the environment for developers that need to do so.
  • [x] cuml_dev_cuda11.5.yml: Remove rapids-build-env and rapids-notebook-env meta packages to reduce size of environment, make upgrading packages easier and have an easy to check explicit list of dependencies.
  • [x] Created new environment libcuml_dev_cuda11.5.yml that only contains C++ dependencies of libcuml++, reducing size of the environment massively for C++ developers.
  • [x] Created new environment libcuml_dev_cuda11.5.yml based on the rapids-build-env and rapids-notebook-env that creates environments that can build any RAPIDS project at the cost of environment complexity and size.

• [x] BUILD.md updates for these and many other recent changes

[codecov-commenter]

Codecov Report

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Flag	Coverage Δ
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[github-actions[bot]]

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[github-actions[bot]]

This PR has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this PR if it is no longer required. Otherwise, please respond with a comment indicating any updates.