Update nightly toolchains in CI (#3097)

This will hopefully fix CI issues we've been seeing with downloads

.github/workflows/main.yml

@@ -58,7 +58,7 @@ jobs:
         submodules: true
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2021-07-12
+        toolchain: nightly-2021-07-18
 
     # Build C API documentation
     - run: sudo apt-get update -y && sudo apt-get install -y libclang1-9 libclang-cpp9
@@ -175,7 +175,7 @@ jobs:
     # flags to rustc.
     - uses: ./.github/actions/install-rust
       with:
-        toolchain: nightly-2021-07-12
+        toolchain: nightly-2021-07-18
     - run: cargo install cargo-fuzz --vers "^0.8"
     - run: cargo fetch
       working-directory: ./fuzz