Probe cargo for -Z json-target-spec support instead of guessing from the version

[athei]

Both PolkaVM build paths compile against a .json target spec produced by polkavm-linker,
so both have to decide whether to pass cargo -Z json-target-spec: newer cargo requires the
flag for a JSON target, older cargo rejects it as unknown.

Both decided by comparing a version against 1.95. pallet-revive-fixtures compared the
rustc version to gate a cargo flag — the wrong binary entirely. wasm-builder compared
the cargo version, which looks right but is not sufficient either: the flag landed partway
through the 1.95 cycle, so a nightly from early in that cycle reports 1.95 from both
binaries while its cargo does not know the flag yet:

rustc 1.95.0-nightly (474276961 2026-01-26)
cargo 1.95.0-nightly (efcd9f586 2026-01-23)   <- no json-target-spec

That is exactly the toolchain in our CI image (ci-unified:bullseye-1.93.0-2026-01-27), so
any job that builds the fixtures on nightly fails:

error: failed to run custom build command for `pallet-revive-fixtures`
  error: unknown `-Z` flag specified: json-target-spec

This is what breaks check-semver on PRs that touch pallet-revive-fixtures (it is one of
the few jobs running nightly rather than the pinned stable, because rustdoc JSON is
nightly-only). It was not hit before because no other PR makes parity-publish rebuild that
crate, and wasm-builder never hit it because its JSON-target path only runs for
SUBSTRATE_RUNTIME_TARGET=riscv.

Since no version comparison can express "does this cargo accept this flag", ask cargo:

fn supports_json_target_spec(&self) -> bool {
    self.command()
        .env("RUSTC_BOOTSTRAP", "1")
        .args(["-Z", "help"])
        .output()
        .is_ok_and(|out| String::from_utf8_lossy(&out.stdout).contains("json-target-spec"))
}

While here, the fixture builder had one more flag on the wrong oracle: a single rustc-version
bool drove both -Cpanic=immediate-abort (a rustc flag) and the
-Zbuild-std-features=panic_immediate_abort fallback (a cargo flag). Those are now separate,
each gated on the version of the binary that parses it. wasm-builder needed no such change:
its CargoCommand::version already comes from cargo --version, and the rustc version is
only used for the "Using rustc version:" printout.

Testing

• Fixtures build clean on stable, contracts actually produced (not a cached skip).
• Under nightly-2026-01-27 — the CI toolchain — the unknown -Z flag error is gone.
• cargo test -p pallet-revive --lib tests::pvm: 137 passed, 0 failed.