Run binaries from ExtraSearchPaths= within tools tree

Run binaries from ExtraSearchPaths= within tools tree

Until now, there was always the implicit assumption that any paths
configured with ExtraSearchPaths= contained binaries built against
the host's /usr. With that assumption, it made sense to execute binaries
found in these paths outside of the tools tree as otherwise you might
end up with missing or out of date libraries if these are not available
within the tools tree.

However, with the introduction of "mkosi sandbox", what I want to do
in systemd is to have contributors build systemd within the sandbox and
then use those binaries in mkosi to build the image. This means that the
build directory configured with ExtrasSearchPaths= suddenly contains
binaries built against the tools tree's /usr (if one is configured) instead
of the host's /usr.

Given this new use case, let's get rid of the logic to not use the tools
tree for binaries in ExtraSearchPaths=, instead, users of ExtraSearchPaths=
using a tools tree will have to make sure to use a tools tree that mostly
matches their host's /usr.