Fix env vars being overwritten in ext unit test script

[Charles-Gagnon]

Env vars such as ADS_TEST_GREP weren't being picked up when running the test-extensions-unit.js script because the script was setting the env vars directly. Changing it to use all existing env vars and then modifying the values it needs for the tests.

[coveralls]

Pull Request Test Coverage Report for Build 2418205338

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.005%) to 41.72%

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Files with Coverage Reduction	New Missed Lines	%
extensions/notebook/src/book/bookTreeView.ts	2	38.21%

Totals
Change from base Build 2418114042:	-0.005%
Covered Lines:	27742
Relevant Lines:	62043

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💛 - Coveralls