helpers/envkernel.sh: return 0 from fish_compat if --fish is not present (MR 2221)

Right now this returns 1 when envkernel.sh is sourced for some reason. I
don't understand why exactly, but this seems like a sensible solution to
me.

Closes https://gitlab.com/postmarketOS/pmbootstrap/-/issues/2133

helpers/envkernel.sh

@@ -405,7 +405,7 @@ main() {
 
 # Print fish alias syntax (when called from envkernel.fish)
 fish_compat() {
-	[ "$1" = "--fish" ] || return
+	[ "$1" = "--fish" ] || return 0
 	for name in make kernelroot pmbootstrap pmbroot; do
 		echo "alias $(alias $name | sed 's/=/ /')"
 	done