Switch sed -E to -r for better portability

GNU sed 4.1.2 does not support the -E flag and this version is used by
some cross-compiling tool chains. Switch -E to -r which is understood.

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13502