Find build-tool installed programs before programs in path

We must consider the path to the installed build-tool before the path to
existing versions of the build tool in paths such as `extra-prog-path`
or in the system path.

This was previously fixed by #8972 but undone by #9527.

This also renames `appendProgramSearchPath` to
`prependProgramSearchPath` to describe correctly what that function
does.

Fixes #9756