plan9port: remove all references to /usr/local/plan9

Fixes #23639.

srcpkgs/plan9port/template

@@ -1,7 +1,7 @@
 # Template file for 'plan9port'
 pkgname=plan9port
 version=20200222
-revision=2
+revision=3
 _githash=92aa0e13ad8cec37936998a66eb728bfca88d689
 archs="i686* x86_64* ppc*"
 wrksrc="${pkgname}-${_githash}"
@@ -28,6 +28,8 @@ post_extract() {
 		-i src/cmd/faces/mkfile \
 		-i src/cmd/mkfile
 	vsed -e 's/-lX11$/& $LDFLAGS/g' -i src/cmd/rio/mkfile
+	sed -i "s,/usr/local/plan9,/usr/lib/plan9,g" \
+		 $(grep -lr /usr/local/plan9)
 }
 do_build() {
 	CFLAGS+=' -D_DEFAULT_SOURCE'