build-unit-test-docker: upgrade googletest to 1.13.0

We've already incorporated a Yocto bump that moves to GTest 1.13,
so we might as well play along in the CI Docker container too.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I44b19e24dc2641ef6e1cba6eea416b4578931c96
1 file changed