Fix swc wasm test on canary (#92738)
## What?
Fixes the failure of "test next-swc wasm" on the canary branch. The env
var it sets incorrectly propagated into the build process for the bundle
analyzer.
Example:
https://github.com/vercel/next.js/actions/runs/24352454644/job/71117953939
This makes sure that the env var is set to empty so that it is ignored
for that step specifically.
Ideally we'd figure out a more granular way to set this env var where it
is needed instead, but this unblocks the CI.
