chore: do not depend on loom on windows (#2146)

Loom currently does not compile on windows due to a
transitive dependency on `generator`. The `generator`
crate builds have started to fail on windows CI. Loom
is not run under windows, however, so removing the
loom dependency on windows is sufficient to fix CI.

Refs: Xudong-Huang/generator-rs#19

tokio/Cargo.toml

@@ -124,10 +124,14 @@ optional = true
 [dev-dependencies]
 tokio-test = { version = "0.2.0" }
 futures = { version = "0.3.0", features = ["async-await"] }
-loom = { version = "0.2.13", features = ["futures", "checkpoint"] }
 proptest = "0.9.4"
 tempfile = "3.1.0"
 
+# loom is currently not compiling on windows.
+# See: https://github.com/Xudong-Huang/generator-rs/issues/19
+[target.'cfg(not(windows))'.dev-dependencies]
+loom = { version = "0.2.13", features = ["futures", "checkpoint"] }
+
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]