Pin grcov version in CI (Qiskit#14198)

The coverage job has been failing the past day or two because the rust
coverage data lcov file can't be found. There was a release of grcov
(the tool used to generate this file) around that time. In an attempt to
unblock CI this commit pins the version installed as part of the job to
the previous release.

Author: mtreinish

.github/workflows/coverage.yml

@@ -31,7 +31,7 @@ jobs:
 
       - name: Install dependencies
         run: |
-            cargo install grcov
+            cargo install grcov@0.8.24
             sudo apt-get install lcov
 
       # This is needed to support any requirements, particularly in the `optionals` set,