Add SHA-256 checksums to the curl-bash-install.sh

[michalrus]

Follow-up to:

• chore: add installers for all platforms #141

Context

Right now we’re not testing that the downloaded archive is what we expect.

HTTP doesn’t validate it on its own.

In the recent GitHub outage, we got a broken archive from them (#139 (review)), so it can happen.

Then, the error was obvious, and it was impossible to unpack the archive, but it could be more subtle.