Correct syntax of shell code in "Check Certificates" workflow

Previously, the command was erroneously split at the first parenthesis. This resulted in the introduction of spaces
between the two parentheses, which caused the syntax to be interpreted as a command substitution operator instead of the
intended arithmetic expansion operator. This caused the command to fail:

```
/home/runner/work/_temp/2470dde2-826c-4a00-8268-b8d111566dc2.sh: line 46: syntax error near unexpected token `/'
```

Author: per1234

.github/workflows/check-certificates.yml

@@ -172,9 +172,9 @@ jobs:
             )"
           fi
 
-          DAYS_BEFORE_EXPIRATION="$(
-            (($(date --utc --date="$EXPIRATION_DATE" +%s) - $(date --utc +%s)) / 60 / 60 / 24)
-          )"
+          DAYS_BEFORE_EXPIRATION="$((
+            ($(date --utc --date="$EXPIRATION_DATE" +%s) - $(date --utc +%s)) / 60 / 60 / 24
+          ))"
 
           # Display the expiration information in the log.
           echo "Certificate expiration date: $EXPIRATION_DATE"