make_chroot: explicitly set path when entering the chroot.

The host system's PATH may not be match the one required by the SDK.
When going through the enter_chroot script it gets reset because bash is
invoked as a login shell but this doesn't happen when using the plain
old chroot command.

Fixes https://github.com/coreos/scripts/pull/290

sdk_lib/make_chroot.sh

@@ -92,7 +92,8 @@ early_enter_chroot() {
 # the overhead of enter_chroot, and do not need access to the source tree,
 # don't need the actual chroot profile env, and can run the command as root.
 bare_chroot() {
-  chroot "${FLAGS_chroot}" "$@"
+  PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" \
+    chroot "${FLAGS_chroot}" "$@"
 }
 
 cleanup() {