ebuild.sh: export EAPI for helpers

This fixes some cases where it wouldn't be exported since commit
20817801dd4ef0117bcc7b33c90650da1e920385.
diff --git a/bin/ebuild.sh b/bin/ebuild.sh
index 2589113..9829f68 100755
--- a/bin/ebuild.sh
+++ b/bin/ebuild.sh
@@ -553,6 +553,9 @@
 
 		[ "${EAPI+set}" = set ] || EAPI=0
 
+		# export EAPI for helpers (especially since we unset it above)
+		export EAPI
+
 		if has "$EAPI" 0 1 2 3 3_pre2 ; then
 			export RDEPEND=${RDEPEND-${DEPEND}}
 			debug-print "RDEPEND: not set... Setting to: ${DEPEND}"