[Process] Fixed #8455: PhpExecutableFinder::find() does not always return the correct binary

Since `PHP_BINARY` is the very first approach considered, there is no way to use a workaround like setting the `PHP_PATH`.

Checking `PHP_PATH` could potentially be put before checking `PHP_BINARY`: that would avoid an extra function call (and system call), at the price of a small BC_BREAK, but I think it's better in this case to have a solution that doesn't force people to set the `PHP_PATH` environment variable.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8455
| License       | MIT
| Doc PR        | none