Don't deny warnings on nightly CI (#1589)

Too much churn!

Closes #1588

.github/actions/install-rust/main.js

@@ -12,8 +12,11 @@ child_process.execFileSync('rustup', ['set', 'profile', 'minimal']);
 child_process.execFileSync('rustup', ['update', toolchain, '--no-self-update']);
 child_process.execFileSync('rustup', ['default', toolchain]);
 
-// Deny warnings on CI to keep our code warning-free as it lands in-tree
+// Deny warnings on CI to keep our code warning-free as it lands in-tree. Don't
+// do this on nightly though since there's a fair amount of warning churn there.
+if (!toolchain.startsWith('nightly')) {
   console.log(`::set-env name=RUSTFLAGS::-D warnings`);
+}
 
 // Save disk space by avoiding incremental compilation, and also we don't use
 // any caching so incremental wouldn't help anyway.