src: update SFI script accessor for V8 8.1

[mmarchini]

The metadata used to access the script field from an SFI changed on V8 8.1. The field is still the same, so no other changes are necessary. This fixes v8 bt on 8.1, as well as stack-test suite.

[mmarchini]

Landed in 8e2a55c82e99

[coveralls]

Pull Request Test Coverage Report for Build ba90ab9ca91d5bbb0a3b1689ae30186deb6973a1-PR-352

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• 4 of 5 (80.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+4.5%) to 78.644%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/llv8-inl.h	2	3	66.67%
<!--	Total:	4	5	80.0%	-->

Files with Coverage Reduction	New Missed Lines	%
src/llv8-inl.h	1	69.65%
<!--	Total:	1		-->

Totals
Change from base Build 1948512b5e71913455f47ff9eb26372762d9b1d0:	4.5%
Covered Lines:	3715
Relevant Lines:	4711

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💛 - Coveralls