Reduce false positives in rust-analyzer compat ci check (#994)

### What
Only consider lines output from rust-analyzer as an error if they begin
with `[ERROR`.

### Why
We've started to see some false positives where the word ERROR appears
in non-error output.

.github/workflows/rust.yml

@@ -45,9 +45,9 @@ jobs:
     steps:
     - uses: actions/checkout@v3
     - run: rustup update
-    - run: rustup +nightly component add rust-analyzer
+    - run: rustup component add rust-analyzer
     - name: Check if rust-analyzer encounters any errors parsing project
-      run: rustup run nightly rust-analyzer analysis-stats . 2>&1 | (! grep ERROR)
+      run: rustup run stable rust-analyzer analysis-stats . 2>&1 | (! grep '^\[ERROR')
 
   build-and-test:
     strategy: