[FrameworkBundle] Fix server run in case the router script does not exist
bug #12489 [2.5][FrameworkBundle] Fix server run in case the router script does not exist (romainneutron)

This PR was merged into the 2.5 branch.

Discussion
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[2.5][FrameworkBundle] Fix server run in case the router script does not exist

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT

I've also added the use of `Process\PhpExecutableFinder`

Commits
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1a79859 [FrameworkBundle] Fix server run in case the router script does not exist