abuild-rootbld: copy REPODEST value from host to chroot

Otherwise, if a different REPODEST is being used (e.g. due to
`buildrepo -d <repo-dest>`) then the abuild invocation in the
created chroot will not write packages to the correct REPODEST.
Therefore, `buildrepo -R -d` does presently not work correctly.
This commit fixes this by also copying the REPODEST value from
the environment.

This fixes a regression introduced in 1582617eb8.

abuild.in

@@ -2504,6 +2504,7 @@ rootbld() {
 		--chdir "$startdir" \
 		--clearenv \
 		--setenv HOME "$HOME" \
+		--setenv REPODEST "$REPODEST" \
 		${ABUILD_BOOTSTRAP:+--setenv ABUILD_BOOTSTRAP "$ABUILD_BOOTSTRAP"} \
 		--setenv SOURCE_DATE_EPOCH "$SOURCE_DATE_EPOCH" \
 		--setenv ABUILD_LAST_COMMIT "$ABUILD_LAST_COMMIT" \