[FrameworkBundle] fix cache:clear command
bug #12999 [FrameworkBundle] fix cache:clear command (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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a14153a [FrameworkBundle] fix cache:clear command