providers: allow parallel installations when not running as a snap

[mr-cal]

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Minor follow up to #4084 to allow check if snapcraft is running as a snap before reading the envvar SNAP_INSTANCE_NAME.

(CRAFT-1660)

[codecov-commenter]

Codecov Report

Merging #4088 (e4137a1) into main (b852f9a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head e4137a1 differs from pull request most recent head 6b5b486. Consider uploading reports for the commit 6b5b486 to get more accurate results

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@@           Coverage Diff           @@
##             main    #4088   +/-   ##
=======================================
  Coverage   94.60%   94.61%           
=======================================
  Files         641      641           
  Lines       51892    51939   +47     
=======================================
+ Hits        49094    49143   +49     
+ Misses       2798     2796    -2     

Impacted Files	Coverage Δ
snapcraft/providers.py	97.72% <100.00%> (+0.05%)	⬆️
tests/unit/test_providers.py	100.00% <100.00%> (ø)

... and 18 files with indirect coverage changes

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