Fix default value in verify-signed-rpms

The value of the `FAIL_UNSIGNED` parameter of the `verify-signed-rpms`
Task is used when calling the `rpm_verifier` binary. An empty string
value is not a valid value result in this error:

```
Usage: rpm_verifier [OPTIONS]
Try 'rpm_verifier --help' for help.

Error: Invalid value for '--fail-unsigned': '' is not a valid boolean.
```

This commit changes the default value to `"false"`. This is both valid
and inline with
[ADR-14](https://konflux-ci.dev/architecture/ADR/0014-let-pipelines-proceed.html).

Signed-off-by: Luiz Carvalho <[email protected]>

task/verify-signed-rpms/0.1/verify-signed-rpms.yaml

@@ -11,7 +11,7 @@ spec:
     - name: FAIL_UNSIGNED
       type: string
       description: "[true | false] If true fail if unsigned RPMs were found"
-      default: ""
+      default: "false"
     - name: WORKDIR
       type: string
       default: /tmp