[build-tools] Let updateEnv override existing values

[sjchmiela]

Summary

• Change BuildContext.updateEnv so incoming values override existing env values.
• Add a regression test covering overrides and clearing via undefined.

Why

This makes updateEnv behave like an update operation and lets callers intentionally refresh or clear existing environment entries.

No idea how it existed so long like this. I guess the values we update with don't overlap with the values we already have that often.

Test Plan

CI should pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.02%. Comparing base (1fcaa05) to head (5c24631).

Additional details and impacted files

@@                       Coverage Diff                        @@
##           stanley/env-allows-undefined    #3901      +/-   ##
================================================================
- Coverage                         59.04%   59.02%   -0.01%     
================================================================
  Files                               935      935              
  Lines                             40974    40946      -28     
  Branches                           8632     8624       -8     
================================================================
- Hits                              24187    24165      -22     
+ Misses                            16691    16686       -5     
+ Partials                             96       95       -1     

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[Copilot]

Pull request overview

Adjusts BuildContext.updateEnv in @expo/build-tools to behave like a true “update” operation by letting incoming env entries override existing ones, and adds a regression test to lock in override + “clear via undefined” behavior.

Changes:

• Changed BuildContext.updateEnv merge order so incoming env values take precedence over the existing context env.
• Added a unit test verifying overwriting an existing env var and clearing one by setting it to undefined.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/build-tools/src/context.ts	Swap spread order in updateEnv so updates override existing env values.
packages/build-tools/src/tests/context.test.ts	Add regression test for overriding and clearing env entries via updateEnv.

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[github-actions]

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