Link with libicuuc, upgrade deps, add CI caching

[anttih]

• Links scheme with libicuuc so that latest purescm works
• Adds CI caching. This shaves off 5 minutes from the build.

[anttih]

Still in CI we get:

Reading Spago workspace configuration...
⚠️ Your project contains a spago.lock file, but it cannot be decoded. Spago will generate a new one.
Error was: $.workspace.packages.build_plan.arrays: Could not decode Lockfile:
  Could not decode WorkspaceLock:
    Could not decode Dependencies:
      No value found

[anttih]

Ah, I have an older spago locally so the spago.lock files don't match.

[anttih]

After updating the lock file with latest spago we get:

Reading Spago workspace configuration...
⚠️ Your project contains a spago.lock file, but it cannot be decoded. Spago will generate a new one.
Error was: $.workspace.packages.build_plan.arrays: Could not decode Lockfile:
  Could not decode WorkspaceLock:
    Could not decode Dependencies:
      No value found

[f-f]

After updating the lock file with latest spago we get

Fixed - we were not using the latest spago, I guess we should fix it in the nix shell