Build libgit2 using git from path

test prepends an always failing git script to the path to fail tests that use git from the path rather than the git binary provided explicitly via the environment. libgit2.a target tries to use this always failing script when building libgit2 as a dependency of test. This commit fixes the problem by using the real git binary that was captured from the user's path prior to overriding it.

Adding only @pks-t as the reviewer as the change is small