fix(luarocks-upload): make sure correct archive is uploaded (#444)

the upload script's `sed` call replaces `'scm'` in the following
section:
```lua
if _MODREV == 'scm' then
   source = {
      url = 'git:https://github.com/nvim-lua/plenary.nvim',
   }
end
```
This PR makes sure it only fixes the declaration at the top of the
file.

scripts/luarocks-upload.sh

@@ -6,5 +6,5 @@ TMP_DIR=$(mktemp -d)
 MODREV=$(git describe --tags --always --first-parent | tr -d "v")
 DEST_ROCKSPEC="$TMP_DIR/plenary.nvim-$MODREV-1.rockspec"
 cp "plenary.nvim-scm-1.rockspec" "$DEST_ROCKSPEC"
-sed -i "s/'scm'/'$MODREV'/g" "$DEST_ROCKSPEC"
+sed -i "s/= 'scm',/= '$MODREV',/g" "$DEST_ROCKSPEC"
 luarocks upload "$DEST_ROCKSPEC" --api-key="$LUAROCKS_API_KEY"