arclight:index_url doesn't fetch remote files

[alexduryee]

When running arclight:index_url with a valid URL, it throws the following error:

rake arclight:index_url URL=https:/path/to/valid/ead.xml
rake aborted!
Errno::ENOENT: No such file or directory @ rb_sysopen - https:/path/to/valid/ead.xml (Errno::ENOENT)

The issue appears to be at https://github.com/projectblacklight/arclight/blob/main/lib/tasks/index.rake#L46, where Ruby's basic open() in invoked (checking the filesystem) and not a HTTP-aware open().