lib/pull: Avoid calling destroy on unref'ed GSource

lib/pull: Avoid calling destroy on unref'ed GSource

We're creating the timer source and then passing ownership to the
context, but because we didn't free the pointer, we would still call
`g_source_destroy` in the exit path. We'd do this right after doing
`unref` on the context too, which would have already destroyed and
unref'ed the source.

Drop that and just restrict the scope of that variable down to make
things more obvious.

Just noticed this after reviewing #1953.