fix: Use proper path for wave fetch. #2177

URL generated for fetching should be of the format: wave-{version}-{operating_system}-{arch}

But the fetch url being generated was using platform instead of the operating_system.

Since this variable is set on the build time. Changed the hatch_build.py file. This will also change the metadata.py file that gets generated.

Closes h2oai#2177

The PR fulfills these requirements: (check all the apply)

[x] It's submitted to the main branch.
[x] When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
[ ] When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
[ ] If changes were made to ui folder, unit tests (make test) still pass.
[ ] New/updated tests are included

h2oai / wave