CLI-19: [self-update] respect proxy settings

[danepowell]

Motivation

Fixes #NNN

Proposed changes

Alternatives considered

Testing steps

1. Follow the contribution guide to set up your development environment or download a pre-built acli.phar for this PR.
2. Clear the kernel cache to pick up new and changed commands: ./bin/acli ckc
3. (add specific steps for this pr)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.99%. Comparing base (460cd47) to head (c201dc7).

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1769   +/-   ##
=========================================
  Coverage     91.99%   91.99%           
  Complexity     1818     1818           
=========================================
  Files           121      121           
  Lines          6834     6834           
=========================================
  Hits           6287     6287           
  Misses          547      547           

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[github-actions]

Try the dev build for this PR: https://acquia-cli.s3.amazonaws.com/build/pr/1769/acli.phar

curl -OL https://acquia-cli.s3.amazonaws.com/build/pr/1769/acli.phar
chmod +x acli.phar

[danepowell]

I have no way to verify that the proxy settings actually work, but I did at least verify there are no regressions.