fix: relax regex for setting environment variables

[benjamintanweihao]

What does this PR address?

This change allows for environment variables with dashes in them.

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• Does the Pull Request follow Conventional Commits specification naming? Here are GitHub's
  guide on how to create a pull request.
• Does the code follow BentoML's code style, both make format and make lint script have passed (instructions)?
• Did you read through contribution guidelines and follow development guidelines?
• Did your changes require updates to the documentation? Have you updated
  those accordingly? Here are documentation guidelines and tips on writting docs.
• Did you write tests to cover your changes?

Who can help review?

@aarnphm

[codecov]

Codecov Report

Merging #2964 (b40c760) into main (f0e0bf0) will increase coverage by 0.89%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2964      +/-   ##
==========================================
+ Coverage   69.71%   70.60%   +0.89%     
==========================================
  Files         104      104              
  Lines        9538     9538              
==========================================
+ Hits         6649     6734      +85     
+ Misses       2889     2804      -85     

Impacted Files	Coverage Δ
bentoml/_internal/bento/build_config.py	67.62% <0.00%> (ø)
bentoml/_internal/utils/__init__.py	77.25% <0.00%> (+0.47%)	⬆️
bentoml/_internal/runner/runner_handle/remote.py	88.54% <0.00%> (+3.12%)	⬆️
bentoml/_internal/utils/docker.py	68.96% <0.00%> (+34.48%)	⬆️
bentoml/_internal/utils/buildx.py	48.29% <0.00%> (+48.29%)	⬆️

[sauyon]

👍 Thanks!