build-unit-test-docker: update ubuntu to hirsute
build-unit-test-docker: update ubuntu to hirsute

The latest Ubuntu "Hirsute Hippo" was released on 2021-04-22.
Switch our CI builds to use it.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: If41dfd2247f0513bc297c4789cb051f7f69cbec4

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts