REGTESTS: use "command -v" instead of "which"

Ilya reported that the "which" utility is not that much portable and is
absent from Fedora. "type -p" is not portable either, and the correct
solution appears to be "command -v", so let's use this for now, we can
change it again in the future in case of problems.

Link: https://www.mail-archive.com/haproxy@formilux.org/msg36332.html

reg-tests/compression/lua_validation.vtc

@@ -40,7 +40,7 @@ shell {
         HOST="\[::1\]"
     fi
 
-    md5=$(which md5 || which md5sum)
+    md5=$(command -v md5 || command -v md5sum)
 
     if [ -z $md5 ] ; then
         echo "MD5 checksum utility not found"