providers: pass SNAPCRAFT_MAX_PARALLEL_BUILD_COUNT to instance

[mr-cal]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run make lint?
• Have you successfully run pytest tests/unit?

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Report from https://bugs.launchpad.net/snapcraft/+bug/1997874

SNAPCRAFT_MAX_PARALLEL_BUILD_COUNT was not being passed into the build environment.

Inside the build environment, the default build count would be used, since the user's environment variable did not exist.

[mr-cal]

Re-targeting from hotfix/7.2 to main because this is not a recent regression and can be part of the next minor release.

[codecov-commenter]

Codecov Report

Merging #3997 (41a8a89) into main (0f49a75) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3997   +/-   ##
=======================================
  Coverage   90.99%   90.99%           
=======================================
  Files          56       56           
  Lines        4154     4154           
=======================================
  Hits         3780     3780           
  Misses        374      374           

Impacted Files	Coverage Δ
snapcraft/providers.py	96.55% <ø> (ø)

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