[FrameworkBundle] fix cache:clear command

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12893
| License       | MIT
| Doc PR        | -

Reading comment on #12893 stating that using absolute paths works around the pb, I tried using realpath(),
and it works!
Pending work on #12955 can be done quietly now.
The path exists because we checked it a few lines above with is_writable()