abuild: restore working dir after updating index

This fixes issue with `abuild cleanpkg all`, which would fail to set
SOURCE_DATE_EPOC for reproducible builds.

abuild.in

@@ -1727,6 +1727,7 @@ update_abuildrepo_index() {
 		DESCRIPTION="$repo $(cd $startdir && $git describe 2>/dev/null || true)"
 	fi
 
+	local oldpwd="$PWD"
 	for i in $allarch; do
 		cd "$REPODEST/$repo/$i"
 		local index=$i/APKINDEX.tar.gz
@@ -1745,6 +1746,7 @@ update_abuildrepo_index() {
 		  mv APKINDEX.tar.gz.$$ APKINDEX.tar.gz \
 		) || (rm -f APKINDEX.tar.gz.$$ ; die "Failed to create index")
 	done
+	cd "$oldpwd"
 }
 
 # predefined function check