add timeout to miri job in CI (bevyengine#4743)

# Objective

- When Miri is failing, it can be very slow to do so

<img width="1397" alt="Screenshot 2022-05-14 at 03 05 40" src="https://user-images.githubusercontent.com/8672791/168405111-c5e27d63-7a5a-4a5e-b679-abbeeb3201d2.png">

## Solution

- Set the timeout for Miri to 60 minutes (it's 6 hours by default). It runs in around 10 minutes when successful
- Fix cache key as it was set to the same as another task that doesn't build with the same parameters

.github/workflows/ci.yml

@@ -73,6 +73,7 @@ jobs:
 
   miri:
     runs-on: ubuntu-latest
+    timeout-minutes: 60
     steps:
       - uses: actions/checkout@v3
       - uses: actions/cache@v3
@@ -83,7 +84,7 @@ jobs:
             ~/.cargo/registry/cache/
             ~/.cargo/git/db/
             target/
-          key: ${{ runner.os }}-cargo-ci-${{ hashFiles('**/Cargo.toml') }}
+          key: ${{ runner.os }}-cargo-miri-${{ hashFiles('**/Cargo.toml') }}
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: nightly