Downgrade Rust toolchain to before Cargo gets a new curl-sys

jrose-signal · jrose-signal · commit 3f515995747b · 2021-10-08T14:05:55.000-07:00
See rust-lang/cargo#9919. The perils of nightly!
diff --git a/rust-toolchain b/rust-toolchain
@@ -1 +1 @@
-nightly-2021-09-19
+nightly-2021-09-16
