minor symfony#43030 Pin ext-redis version in CI (derrabus)

This PR was merged into the 4.4 branch.

Discussion
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Pin ext-redis version in CI

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Backport from symfony#43029
| License       | MIT
| Doc PR        | N/A

Commits
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ca34753 Pin ext-redis version in CI

Author: derrabus

.github/workflows/unit-tests.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: Ubuntu-20.04
 
     env:
-      extensions: amqp,apcu,igbinary,intl,mbstring,memcached,mongodb,redis
+      extensions: amqp,apcu,igbinary,intl,mbstring,memcached,mongodb,redis-5.3.4
 
     strategy:
       matrix: