fix git http_proxy ignored #5277

Job	Run time
prepare	1s
test / prepare	2m 37s
test / run (containerd, ./frontend/dockerfile, integration)	5m 6s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 3s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 9s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	7m 8s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 14s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 39s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	5m 13s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 10s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 23s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 31s
test / run (oci, ./frontend/dockerfile, integration)	4m 43s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 13s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 54s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 49s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 51s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	6m 41s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 47s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 26s
image (mainline)	5m 37s
image (labs)	5m 26s
scout	0s
release	0s
	2h 0m 41s

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