[integration-tests] Update randomStringGenerator bash function

Some integration tests workflows failed with:
```
Error: Invalid format 'ORG_WEBHOOK_SECRET=***'
```
or
```
Error: Invalid format 'GARM_PASSWORD=***'
```

Workflow runs logs:
* https://github.com/cloudbase/garm/actions/runs/5920389694/job/16051606203#step:7:26
* https://github.com/cloudbase/garm/actions/runs/5908193226/job/16027297143#step:7:26

This is a transient error, as it only happens sometimes.

I suspect that sometimes there is some illegal sequence of characters
in the random generated strings. Thus, the GitHub actions logic to
parse the environment fails.

This change removes the special characters that would have a special
meaning in bash, from the `randomStringGenerator` function, in hopes
to fix the transient issue.

Signed-off-by: Ionut Balutoiu <ibalutoiu@cloudbasesolutions.com>

.github/workflows/integration-tests.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Generate secrets
         run: |
           function randomStringGenerator() {
-            tr -dc "a-zA-Z0-9@#$%^&*()_+?><~\`;'" </dev/urandom | head -c 64 ; echo '';
+            tr -dc "a-zA-Z0-9@#%^&*()_+?><~;" </dev/urandom | head -c 64 ; echo '';
           }
 
           GARM_PASSWORD=$(randomStringGenerator)