meta: omit LD_LIBRARY_PATH and PATH in classic confinement

[mr-cal]

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When confinement is classic, do not use the default values for LD_LIBRARY_PATH and PATH.

Includes an optimization/refactor suggestion from my original implementation: #3758 (comment)

Resolves #4187

(CRAFT-1774)

[cmatsuoka]

Thanks!

[codecov-commenter]

Codecov Report

Merging #4216 (1039432) into main (64c8f1e) will decrease coverage by 0.02%.
The diff coverage is 72.41%.

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@@            Coverage Diff             @@
##             main    #4216      +/-   ##
==========================================
- Coverage   93.90%   93.88%   -0.02%     
==========================================
  Files         569      569              
  Lines       43746    43763      +17     
==========================================
+ Hits        41078    41089      +11     
- Misses       2668     2674       +6     

Impacted Files	Coverage Δ
tests/unit/meta/test_snap_yaml.py	96.75% <63.15%> (-3.25%)	⬇️
snapcraft/meta/snap_yaml.py	97.03% <90.00%> (+0.39%)	⬆️

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[AlanGriffiths]

I realise this has merged, but this seems the best place to call out problems. Many of the classic snaps I maintain rely on having executables on $PATH (typically $SNAP/bin or $SNAP/usr/bin).

I doubt that I'm the only one using the existing behaviour of adding these to $PATH. While I have sympathy for the change dropping them (my snaps need logic to strip them) this is probably going to break a lot of snaps if promoted in the current form.