buildman: Put the toolchain path first instead of last in PATH

If:

1. Toolchains A and B have the same filename
2. Toolchain A is in the PATH
3. Toolchain B is given in ~/.buildman and buildman uses it to build

then buildman will add toolchain B to the end of its path but will not
necessarily use it since U-Boot will find toolchain A first in the PATH.

Try to fix this by putting the toolchain first in the path instead of
last.

Signed-off-by: Simon Glass <sjg@chromium.org>