Pin an older nightly to fix wasi tests (#671)

Rust's recent update to libstd of the wasm32-wasi target turned out to
be buggy with respect to fetching the process arguments, so we'll need
to wait on a fix there before we can run these tests with nightly again.

.github/workflows/main.yml

@@ -102,9 +102,11 @@ jobs:
           - build: beta
             os: ubuntu-latest
             rust: beta
+          # FIXME need to wait for rust-lang/rust#67065 to get into nightlies,
+          # and then this can be updated to `nightly` again.
           - build: nightly
             os: ubuntu-latest
-            rust: nightly
+            rust: nightly-2019-12-03
           - build: macos
             os: macos-latest
             rust: stable