makedist: fix rockspec tag update producing invalid Lua

The sed that sets the source tag inserted a new line after the url
line without a trailing comma and without removing a pre-existing
tag line. When a release is cut from a tree that carries the previous
release's rockspec (which already has a tag line), the result is a
syntax error:

   source = {
      url = "git+https://github.com/luarocks/luarocks",
      tag = "v3.13.0"
      tag = "v3.12.2",
   }

This is what shipped in the official luarocks-3.13.0.tar.gz (#1851).

Fix by deleting any existing tag line before inserting the new one
(with a trailing comma), and validate that the rockspec actually
loads as Lua before packing the tarball, using the Lua 5.4 prefix
the script already requires.

0 files changed, 0 insertions, 0 deletions