For starters, rstudio.cloud does not provide an SMTP server with which to send emails. You will need to connect to your own SMTP server, or a service that provides one, such as Google, Mailgun or Sendgrid.
It appears that blastula allows mailsend-go to be located in the current working directory, so you may be able to grab the tarball and extract it in your project.
I believe that there are other R packages for sending email that might not require additional binaries.