prereq-build: fix inconsistent value of $PATH

prereq-build: fix inconsistent value of $PATH

In the recipe SetupHostCommand for checking
and creating symlinks, $PATH was only overridden
for one of several commands.

This causes the symlinks to be included
in the paths to pick a program from
when the check was repeated, because
staging_dir/host/bin was included in $PATH,
but only *sometimes*.

When the check ran again, the command succeded
with a $PATH including the symlink,
(eval "$$$$$$$$cmd")
while the path to the program was evaluated
with a $PATH NOT including the symlink,
(bin=...)
causing the symlink to be relinked incorrectly,
instead of passing as exactly the same.

Coincidentally, this was only a problem
if the symlink targeted the alternative
program with a different name.

By overriding the value of $PATH at the invocation of Make,
it will apply to the entire environment of the checks.

Signed-off-by: Michael Pratt <mcpratt@pm.me>