release: unset SKIP_WASM_BUILD (#7074)

# Description

Seems like I added `SKIP_WASM_BUILD=1` 💀

 for arch64 binaries, which
results in various errors like:
https://github.com/paritytech/polkadot-sdk/issues/6966. This PR unsets
the variable.

Closes #6966.

## Integration

People who found workarounds as in #6966 can consume the fixed binaries
again.

## Review Notes

I introduced SKIP_WASM_BUILD=1 for some reason for aarch64 (probably to
speed up testing) and forgot to remove it. It slipped through and
interfered with `stable2412` release artifacts. Needs backporting to
`stable2412` and then rebuilding/overwriting the aarch64 artifacts.

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Signed-off-by: Iulian Barbu <[email protected]>