Set PL_BACKEND in L-GPU docker build and misc improvements

[vincentmr]

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---

Context:
The Lightning-GPU Docker image has not installed the wheel correctly as the wheel-building command was missing the environment variable PL_BACKEND=lightning_gpu.

Description of the Change:
Set the environment variable PL_BACKEND=lightning_gpu in the build-wheel-lightning-gpu Docker-build stage.
Enhance Docker-related make commands.
Enable Dockerfile level defaults for build arguments.
Set the default Lightning and PennyLane versions to master.

Benefits:
L-GPU installs the correct wheels.
No need to update hardcoded tags/versions since master is moving.

Possible Drawbacks:

Related GitHub Issues:
[sc-68127]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.11%. Comparing base (be07bbe) to head (632c35d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
- Coverage   94.95%   93.11%   -1.84%     
==========================================
  Files         132       17     -115     
  Lines       19542     1904   -17638     
==========================================
- Hits        18557     1773   -16784     
+ Misses        985      131     -854     

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[multiphaseCFD]

Thanks @vincentmr!

[AmintorDusko]

Looks good!