### What does this PR try to resolve?
This is a horrible hack,
which lets the rustc invocation for learning target info always emit
warnings as warnings.
But at least it unblocks people who pass `-Awarnings` via RUSTFLAGS.
A long-term solution is a better interface
between build systems and the compiler.
See the discussion on Zulip:
https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Improving.20communication.20interface.20between.20cargo.2FrustcFixesrust-lang/cargo#8010
### How should we test and review this PR?
Ensure `CFG_DISABLE_CROSS_TESTS` is not set,
and run `cargo t --test testsuite
always_emit_warnings_as_warnings_when_learning_target_info`
This also additionally adds `wasm32-unknown-unknown` target to Cargo's
CI.
### Additional information
Instead of allowing any type in metadata, we were specifying fields like
`"string": "<any string>"`.
Found this when looking into #15030
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I found a bug in the manifest parser and figured this would help make it
more obvious.
Since I was already changing the order, I figure I'm make things a
little more logical (user-facing first, implementtion details later)
