Fix elements naming in gitian builds

[psgreco]

Currently, our gitian builds for releases generate tarballs called elements-elements-xxxxx instead of elements-xxxxx. This is fixed by correctly trimming the elements- prefix in our release tags (just as bitcoin trims the initial v)

[psgreco]

utACK c6677f8 this change will remove the first instance of elements- in the RECENT_TAG var

yes, bitcoin tags their releases with vXXXX, we tag them with elements-XXXX, this patch strips the elements- prefix the same way bitcoin removes their v