fix(wasm): Remove the CMAKE_STAGING_PREFIX variable from wasm preset

[phated]

Description

This removes the CMAKE_STAGING_PREFIX variable from the wasm prefix. This was a holdover from the wasi-sdk toolchain file. I originally left it in because I didn't know what it did; however, I've discovered that this value is used as the install prefix if it is set (see https://discourse.cmake.org/t/cmake-staging-prefix-vs-cmake-install-prefix/6485) and that makes the wasm file install into the wasi-sdk sysroot, which is wrong for our project.

Removing this allows the wasm file to install into the same prefix as the rest of the project.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[kobyhallx]

👍 LGTM