supply libgit with flag to respect env variables. #7996

Job	Run time
build-linux-arm (nightly)	6m 57s
build (ubuntu-latest, nightly)	2m 49s
build-apple-x86 (nightly)	2m 57s
build-linux-musl (nightly)	3m 59s
build (ubuntu-latest, stable)	2m 35s
build-linux-arm (stable)	6m 37s
build-apple-x86 (stable)	2m 32s
build-linux-musl (stable)	3m 30s
build (ubuntu-latest, 1.70)	2m 46s
build-apple-x86 (1.70)	2m 46s
build-linux-arm (1.70)	6m 53s
build-linux-musl (1.70)	3m 40s
build (macos-latest, nightly)	2m 58s
build (macos-latest, stable)	2m 31s
build (macos-latest, 1.70)	4m 5s
Lints	34s
udeps	2m 12s
build (windows-latest, nightly)	5m 11s
build (windows-latest, stable)	6m 4s
build (windows-latest, 1.70)	6m 29s
Changelog Test	5s
	1h 18m 10s

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