Refactor setup.py to align with CF.

[vincentmr]

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Context:
The way setup.py is written, it tries to find binaries in the environment (for MacOS in particular) and it may override some CMake decisions which would actually make sense in some build environments (e.g. on Conda-Forge). This is trying to apply this patch and a bit more.

Description of the Change:
Remove (most of ) CMake argument parsing from setup.py. Adapt CI files and use env: when possible to set environment variables.

Benefits:
Where necessary, libraries and default build tools are defined declaratively in the Actions files. setup.py is simplified and any CMake argument can be passed in CMAKE_ARGS if required. This complies better with Conda-Forge, and hence we'll be able to remove some patches there.

Possible Drawbacks:
Building from source may change a bit for users used to do so.

Related GitHub Issues:
PR 58

[codecov]

Codecov Report

Merging #450 (fb5cb7d) into master (a83875a) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
+ Coverage   99.76%   99.82%   +0.06%     
==========================================
  Files          42       50       +8     
  Lines        3811     4667     +856     
==========================================
+ Hits         3802     4659     +857     
+ Misses          9        8       -1     

Impacted Files	Coverage Δ
pennylane_lightning/_version.py	100.00% <100.00%> (ø)

... and 41 files with indirect coverage changes

[AmintorDusko]

Some suggestions.

[mlxd]

Thanks @vincentmr
Just a few thoughts and comments

[mlxd]

Thanks @vincentmr

[AmintorDusko]

Looks good to me!