sdk: repin to fizzy SDK 0.2.31

[github-actions]

Repins the plugin from fizzy SDK 0.2.30 to 0.2.31, because the build fails against it.

• build.zig.zon: the sdk-v0.2.31 release asset and its hash
• plugin.zig.zon: .version → 0.2.38, .min_sdk_version → 0.2.31

It does not build against the new SDK yet, so this is a draft: the plugin needs changes on this branch first. The end of the build:

./build.zig.zon:21:21: error: hash mismatch: manifest declares fizzy-0.2.31-SGBwY8J_JwCennA3idgxGcp2-Cafv6x6o_UB_TYEWql2 but the fetched package has N-V-__8AAMJ_JwBv41TF4cOd_sM0n6bG5nSuY95WRGwpiLqJ
            .hash = "fizzy-0.2.31-SGBwY8J_JwCennA3idgxGcp2-Cafv6x6o_UB_TYEWql2",
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Opened by fizzyedit/plugin-build-action's repin workflow.