Hmm I did testing on Mandriva 2008.1 and it worked for me, however I do have /sbin and /usr/sbin in my PATH (I don't remember editing it, but that doesn't mean I haven't).
The solution in any case is actually simpler than writing the whole path to urpmi, rather just start su with "-" to indicate a login shell.
I should point out that this will not prevent you running phoronix-test-suite, it will only prevent you installing any test dependencies automatically.
diff --git a/pts/distro-scripts/install-mandrivalinux-packages.sh b/pts/distro-scripts/install-mandrivalinux-packages.sh
index 6d1b46a..5db7f52 100755
@@ -2,5 +2,5 @@
# Mandriva package installation
-su root -c "urpmi --auto $@"
+su - root -c "urpmi --auto $@"