golang: Move build cache into $(TMP_DIR)

This changes the default build cache directory from $(TOPDIR)/.go-build
to $(TMP_DIR)/go-build, so that the cache directory is ignored by git
and is removed by "make dirclean".

This does not move/remove a build cache directory in the previous
location; it will need to be removed manually.

Fixes https://github.com/openwrt/packages/issues/13675

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Signed-off-by: Jeffery To <jeffery.to@gmail.com>