machinst x64: add testing to the CI;

crates/wiggle/macro/Cargo.toml

@@ -14,6 +14,12 @@ include = ["src/**/*", "LICENSE"]
 proc-macro = true
 test = false
 
+# This has been added to make testing of the experimental_x64 feature possible,
+# working around a Cargo nightly bug: once
+# https://github.com/rust-lang/cargo/issues/8563 has been fixed, we should be
+# able to revert it.
+doctest = false
+
 [dependencies]
 wiggle-generate = { path = "../generate", version = "0.19.0" }
 witx = { path = "../../wasi-common/WASI/tools/witx", version = "0.8.5" }