Fix `wasm-builder` not exiting if compilation fails (#3439)

This PR fixes a subtle bug in `wasm-builder` first introduced in
https://github.com/paritytech/polkadot-sdk/pull/1851 (sorry, my bad! I
should have caught this during review) where the status code of the
`cargo` subprocess is not properly checked, which results in builds
silently succeeding when they shouldn't (that is: if we successfully
build a runtime blob, and then modify the code so that it won't compile,
and recompile it again, then the build will succeed and silently use the
*old* blob).

cc @athei This is the bug you were seeing.

[edit]Also fixes a similar PolkaVM-specific bug where I accidentally
used the wrong comparison operator.[/edit]