Lock CI rustc to working nightly (https://github.com/rust-lang/rust/issues/68264)

.github/workflows/build.yaml

@@ -24,10 +24,10 @@ jobs:
         submodules: true
     - uses: actions-rs/toolchain@v1
       with:
-        toolchain: nightly
-        override: true
+        toolchain: nightly-2020-01-14
+        profile: minimal
+        default: true
     - uses: actions-rs/cargo@v1
       with:
         command: test
-        toolchain: nightly
         args: --release ${{ matrix.features }}