Make GITHUB_URL overridable

[unguiculus]

Proposed Changes

Make GITHUB_URL in install.sh overridable.

Types of Changes

New Feature

Verification

Set GITHUB_URL. Can just be set to the default URL for testing to verify the override works. Alternatively, using a file:// URL also works.

Linked Issues

#11691

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.65%. Comparing base (0d028a2) to head (e0c3758).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11692      +/-   ##
==========================================
- Coverage   49.90%   47.65%   -2.26%     
==========================================
  Files         185      185              
  Lines       19353    19353              
==========================================
- Hits         9658     9222     -436     
- Misses       8302     8797     +495     
+ Partials     1393     1334      -59     

Flag	Coverage Δ
e2etests	40.37% <ø> (-3.75%)	⬇️
inttests	35.05% <ø> (+<0.01%)	⬆️
unittests	17.03% <ø> (-0.01%)	⬇️

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